this is my opinion, without knowing any other information about what your trying to do:

vampire:
pros:

• dont lose access to current skills
• more skill and build options
• undeath
• ulti gen from vamp drain

cons:

• regen penalty, resist penalty (fire), and making non vamp skills cost more
• most passives and skills are not that useful for a tank outside of undeath, especially making things cost more

WW
pros:

• stamina regen while transform slotted
• transformed WW has more tank options (taunting with heavy attacks with specific morph of the roar)

cons:

• need specific build for when transformed as you lose access to all of your normal skills and limited to options within WW skill line
• negative resistance to poison (only while transformed)
• gains no extra block mitigations (i dont remember if WW inherit weapon passives while transformed, i dont believe they do though)

off the top of my head at least

to be honest as a tl;dr i would say that neither vamp or WW is great for PVE in general unless your going for a specific theme build

for vamp its mostly the -recovery of higher stages and increased cost of other abilities

there are some niche cases for being a vampire to access the vamp drain as a tank as the ult gen is good, mainly can work as an off tank in some trials, and in some cases in archive (like a perma magma shell DK setup), its hard to use as a main tank because of the channel you have to be ready to cancel it for blocking

if you want to be vamp i would probably suggest staying at stage 1 so you dont get penalized much but still have access to the skills