I'm just wondering if there's any downside to starting a new game with both DLC installed? For example I heard about the mysterious traveler who kills NPC's.

I don't want a single NPC to die from this mysterious traveler, so is it better to install Dawnguard later?

I heard about vampires killing off whole towns with Dawnguard installed. But it doesn't seem to happen if you do the Vigil of Stendarr quests. (Not sure which ones. I went to their base and did the quest in Markarth and it seemed to work.)

The mysterious traveller only spawns in the cell your character is in, and you're the only person he targets. He won't kill NPCs.

There's still the issue of vampire attacks, though. They don't stop even if you finish the Dawnguard questline and are similar to dragon attacks - vampires arrive in towns, NPCs get caught in the crossfire, shopkeepers die etc. If you're not prepared to face them, you should wait before installing Dawnguard.

If you're on PC, there's a mod disabling random vampire encounters.

Don't they only attack when you fast travel?

So if you never fast travel you should be fine I believe?

I've heard the opposite.

Isn't this related to Dawnguard? Just saying.

They seem to show up more in towns for me when I fast travel. Otherwise you encounter them more often in the countryside while traveling. But it's not a sure thing that they will never appear in a city if you never fast travel.

I think fast traveling to a stable and walking to the nearby town gets rid of vampire attacks.

I always thought they appear if it's dark and you're in town so i try to keep my visits limited to daylight time.

Most of the above is wrong....they can attack if you don't fast travel, they can attack during daylight hours, and they don't always stop after you complete the DG main quest. Different ppl seem to have different experiences....I think there is a bug (s) involved because it varies wildly for me...some characters get few attacks, some get a ton

Edit: the above applies to random vampire attacks, not mysterious stranger