Hi, I have a character that has become a vampire and is now controlled by the DM (me), but I don’t know how to add this character to the encounter builder? In general it would be useful to add NPCs to encounters.

Any tips?

There are two ways to handle NPCs in both D&D in general and DDB specifically:  build them as PC characters, or build them as "monsters."  The official resources do the latter, and that's what DDB expects you to do.

You can build them as PCs and add them to the encounter builder here, but:

1. they have to be added to the campaign, and players will at the very least be able to see the info on their "badge" on the campaign page, and more if you don't set their privacy setting to private
2. DDB will count them toward the party's side of the encounter difficulty calculation.  One workaround for this is to exclude them from the party initially as you build the encounter, until you have the difficulty set where you want it, and then add them.  (The excluding and adding happens from the "Manage Characters" button in the Encounters tool)

If you build them as monsters, they are calculated as a enemy in the encounter difficulty calculations.

I actually have the opposite problem from you, as I have homebrewed "monsters" to represent a sidekick and a familiar.  I have to build the encounter without them, and only add them when the encounter looks as it should.  (And, for that matter, use a fill-in PC character to stand in for the sidekick when calculating difficulty)

And, before you ask, one of the issues with not being able to assign "sides" to NPCs built as PCs or to "monsters" who are allies is that there is no official way to convert levels to CR or vice versa. 

If it were me I would convert it to a monster statblock. But that’s me. If you want to and need help I can help you.