Do you use shouts ? The slow time shout is a god send for archer characters. Marked for death is another non-dawnguard related shout that is a benefit, I would as well try and ditch Serana as often as you can. She's not the best of followers when it comes to combat ( my opinion ), I would go with a " heavy " follower, one that skilled in heavy armor. Deck them out in top notch gear and your butt is pretty much covered, or you could get a " ranger " class follower like Aela and once again deck them out in top notch gear. Another good tip is to have a dog with you as well ( if you can put up with the dogs ) this way you can tell both your dog and your follower to wait at the main gate seeing how this is where the chances of a vampire attack is greatest. In doing so you have your own " guards " watching the gate along with the towns guard, no need to worry too much about the attacks then all because your followers and the towns guard should be enough to take care of it, but I would still keep an eye on everything just in case you need to steep in.

I once had Jenassa stationed at the main gate in Whiterun just in case some vamps showed up. I came out of breezehome and sure enough there was a vamp attack, but Jenassa just stood there with her back to all the commotion and did nothing until I rushed in to help. Maybe this was because she was facing the wrong way. But it wasn't like this was a stealth attack.

That's odd

Lets use Whiteruns maingate for an example. I will post my follower right at the end of that little bridge just as you enter the town, just before Warmaidens and they always engage the vampires. Sometimes my follower has a vampire or two killed long before the towns guard can react, even Janessa acts the same for me. Perhaps you said something or did something that pissed her off

Bow with Fortify Marksman potion, or swallow an invisbility potion and slit their throat.

At the Whiterun gate in my game are two Alik'r warriors that I reverse pick pocketed Legendary Scimitars enchanted with high level Turn Undead (and took the ones they had), and they were the most awesome Vampire dispatchers in the game. I now wish more of those Alik'r guys had gone looking for Saadia in other cities.

A follower may not attack anything unless you are engaged in the fight (or maybe at least have your weapon drawn/spell ready).

A good reason to resolve the quest in Saadia's favor. Then you get two permanent gate guards.

Well I got the vampire amulet mod, and I don't regret it. Back to doing quests and roleplaying.

That's a pretty damn smart idea I may have to borrow it if you don't mind
The follower thing you mentioned..... I never have to worry, every follower that's I've ever used since Dawnguards release has always defended the main gate of what ever town/city I visit. I don't even need to be close to the main gate, there's been a couple times now that I'll on the other side of the city conducting business and yet my follower will still defend the gate, I've seen it in progress so I joined in on the fun

I've never tried using a follower like that, but are you a Vampire? I don't get Vampires attacking towns any more, but when we see them being attacked by something else in the wilderness, Marcurio never goes instantly aggressive like he does for a bear, or something. I kinda figured it's because we are both Vampires, so we end up killing whichever side I choose (usually the Vampires, since we are territorial ).

I do still get frequent attacks in town by the Dawnguard - and if I'm not careful citizens will come to my aid and get killed. I wonder if stationing Marcurio would help with this?

I get a lot of Dawnguard attacks at Lakeview Manor, and none of the house staff have ever attacked them before they attack Braggen, not sure if a follower is going to be smarter since I hardly ever have one. Braggen has taken to conjuring up a Dremora Lord, since he yells real loud as soon as he sees anything and goes for it, I am sick of things sneaking up on me, even the dragons are starting to get a bit sneaky.

There is also....Dawnbreaker

My follower Marcurio will always go into combat mode when there are Dawnguard near, but not for Vampires unless they directly attack us. I figured this is because I have completed the Vampire faction, and maybe that's the reason why Jenassa didn't attack the Vampires for cap_and _gown. Whenever the Dawnguard spawn in a town they come straight at me, and I'm still not 100% sure if they deliberately kill the citizens who come to defend me, or if it's some kind of accident. The Dawnguard will attack and kill the town guards, though, so they're not so lawful good as they pretend.

No I sided with the Dawnguard and will only use a non-vampire follower Still my follower/companion will do a marvelous job guarding the main gate as I go about my business, you should give it a try, it relieves a good portion of the stress when entering a town/city. It's only the open towns that don't have walls that stress me out all because the vampire attack can happen at any time and come from any direction so I tend to keep my follower next to me when I visit a town like Riverwood.

Can't you enchant arrows with fire damage?

I prefer to cut them to pieces with my flame-enchanted ebony sword. I may as well marry the damn thing.

Huh? What type of armor do you wear? I actually smithed the Dawnguard armor up to legendary and then placed 2 enchantments on armor, gauntlets and boots. Even if you don't have the top Enchanting perk, at your level, you should know a few enchantments. Use the fortify heavy or light armor enchantment (depending what you wear), and get some disease resistance on there. Did you upgrade your weapons? I'm using Dragonbone weapons and the vampires don't stand a chance.

I'm not a mage, and I usually don't play mages. Plus, travel with a companion. Oh and there are plenty of potions you can take, when vampires attack... fortify health, fortify one-handed, fortify light armor etc etc etc...

Oh as for Serana, you need to give your followers some equipment LOL Serana is either wielding a Staff of Fireballs or a Staff of Ice Spikes in my game...

If you have Hearthfire and chose the kitchen wing, you can make garlic bread in the oven which also cure diseases. It only weights .1 as well. That's what my Dawnguard vampire hunter is carrying.

Dawnbreaker and crossbows.