CR 12 lvl 10 wizard/ vampire?

[Rasmus]

Probably a lightningbolt, since chainlightning is level 6, and call lightning is a 3rd level druid spell.

Or he might have used a scroll...

 

[Erithtotl]

It sounds like there are a few possibilities.

1) There is some 'special' way to defeat this vampire, some item or method you can research to defeat him.

2) He is supposed to be a thorn in your side until you get more powerful and defeat him.

3) Bad DMing.

I don't know your DM, but from your descriptions #3 seems kinda likely. The battle you described where he showed up, reduced most of the part to near death, and then left made no sense. Why didn't he just wade in and wipe you all out? As others have mentioned, a vampire that powerful could easily wipe out a party of your description with minimal effort.

 

[Tidus4444]

Actually, the reason why he left is that he was at 2 hp. We did pretty good against him. Oh, and it wasn't near death. He just Lightning Bolted me twice and reduced me to -6. Aside from the con damage from Cloudkill, everybody else was at full HP.