Sadly when I transform into a Vampire Lord I get attacked by everyone does this keep happening or is a glitch?

Also I after completing the main quest line for either side will night attacks stop? The fact I might lose NPC's I might just not install Dawnguard at all.

This happened to me as well, I have learned to not transform in the Castle. Im not sure if it is a glitch or somthing to do with the transformation itself.

Cheers

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... indeed.

Previously, with the vampirism stages, once stage 4 you would be attacked on sight by any NPC, as they recognize your true nature. But with Dawnguard it was changed, you no longer are attacked at stage four, it's only if you are a vampire lord and you switch to that form, then people recognize your true nature and you will be attacked on sight, however this only applies outside of castle volkihar.

It's the same mechanic used for werewolves, you are attacked on sight for being in your werewolf form, but if outside of the form, then no one is any the wiser.

As for vampire attacks, no. They never stop. I personally use a mod that makes it so the attacks never start until after you start the dlc quests by joining the dawnguard and it also disables town attacks, cause really, who wants their towns to eventually only contain immortal npcs? It does not stop any of the random wild attacks, only the town ones so I can still enjoy the random vampire encounters outside of town, where they aren't one shotting the civilians.

If people inside the castle are attacking you for switching, I would count that as a bug. I haven't experienced it but I'm only recently coming back after not playing since august, where I played a lot of dawnguard then took a break again. Back then they would just say such things like "you think you are impressing anyone here with that?" Outside of the castle that is working as intended however.I hope this answers your question.

It's a glitch as the vampires should accept you in the castle. The vampire form works like the werewolf form around towns and stuff, but is completely accepted in the castle volkihar assuming you're sided with them, unless you have a mod which casts a spell upon transformation assuming you're on PC, if not than it is just simply a glitch.

Yes attacks do stop when you finish the questline on either side, assuming you do not run into the bug that keeps the attacks happening.

If need be, get your levels you want, grab all the stuff you need (weapons, potions, etc) and then install.