I'm not sure what is hanging it up, but the belt tensioner does not have to be removed to get the vac pump off. You may need to push the tensioner all the way up or down (forget which), and/or rotate the engine slightly so the vac pump is not under tension from the timing device cam. I just did this job last week and the tensioner remained in place (including the shock).

Just remember to set the engine at ~14° ATDC, and insert the IP lock tool (making sure it's connected to the tang), before removing the pump. In addition to the side, bottom, & rear covers, ELR and vac shutoff... don't forget to also replace the stop lever shaft O-ring. This is often forgotten and often leaks!