Is this the main vac part, or the pump for the wet part?

Erm, not sure I understand the question. It's the big black circular heavy thing with a motor in it and a flex going to it that sits on the top of the body in which the bag sits.

To convert it from wet to dry you take out the bag and filter and put in another filter with a float valve.

As far as I can see the thing that does the sucking is the same whether it's in wet mode or dry mode - in wet mode it sucks water and in dry mode it sucks dirt!