What is the best vendor (and difficulty) to get a wand with life tap charges? Ideally the highest level life tap that I can get.

Drognan, find a map where he's located closed to town entrance to, and just run in/out.

You can get up to a lvl 6 life tap wand Iirc with a high enough character shopper

Alright. Thanks for the reply.

One last thing. Would you recommend doing this in Hell difficulty or what?

NM/Hell doesn't matter, ilvl item = clvl +5.

but in norm , vendor ilvl are hardcoded i think

Yeah, Drognan on NM or Hell with Town exit next to him is the best spot to shop them.
You need at least a lvl 91 shop-char to get the lvl 6 life tap. Else the lvl 5 what makes not a big difference you can already get at shop-char lvl 78.

~LIFE TAP WAND UPDATE~

I am pleased to announce that I have found a good life tap wand from Drognan like you guys suggested. Thanks!