"Anyway - are there ways / plugins to achieve a smoother / dreamier motion blur effect in Vegas than the one thatäs available as default?"

For video I've never found any, for animation done in Vegas yes.

You could try changing the type of motion blur that Vegas is using, that's set in the Project settings but you're correct, all it's doing is combining frames. You can change how it does it, that's all.

One idea just entered my head, worth a try. Speed the footage up say 400%, apply lots of MB. Render that out and then take the rendered file and slow that back down 400%.

Bob.