The problem is that everyone who ever stalled or nearly stalled an aircraft using VS at high altitude thought there was no problem as they were perfectly capable of minding the shop...
As the OP says, if the a/c doesn't want to climb then you're probably asking too much, either in altitude or climb speed.
As for being concerned at the RoC varying with a few knots speed change, I've never understood why this is seen as a problem. The attitude rarely varies more than a degree or two.