Just an update: Steam version update didn't stop the fps drops, nor did changing a bunch of graphics options to the lowest I could set them (and/or "off").
However, VHVR released a new update last week and THAT stopped the major fps drops and lag/stuttering I'd been getting. I was even able to add on to my base (including a bunch more land changes) with no problems. Dunno if it was an issue with the previous version of VHVR getting messed up by the July 1 patch, or if they added some game optimizations that fixed things, but either way, problem seems to be resolved. Apart from the game itself not being all that well optimized to begin with, but like I said, playable for me again. In VR.