how to move the 'New MacVim Buffer Here' menu into the main context menu?
I don't think there is a "clean" way to specifically move a single service item in the context menu. 

What you can do is increase the amount of items allowed in the context menu before a submenu is created.

In Terminal.app:

defaults write -g NSServicesMinimumItemCountForContextSubmenu -int 7
killall Finder


This will allow up to six elements in the top level menu. Substitute 7 with the desired number.

Unfortunately, all elements from your submenu will be moved up, not just the MacVim one.