As a follow-up, upgraded the HomeSeer dimmers to firmware 5.19 and it now works great. Before the firmware upgrade, connection would be spotty. After the upgrade, connection is solid. Feedback works great, so scenes on Vera triggered by these dimmers execute almost instantly.
However I still have issues with the HomeSeer switches. Out of three switches on the same room, all with the same firmware 5.14, one switch works great with instant feedback. Another switch, which is on the same box as the one working well, barely communicates with Vera, who's showing connection errors. Another switch works most of the time but has to be polled, no feedback and associations don't seem to work either.
To upgrade, got a Z-Wave.me UZB ZWave plus USB interface (works great), but HomeSeer, which is a windows .net app, did not work well on Linux (It would start but could not download the Z-Wave plugin, which is optional), and worked more or less unreliably on a Windows VM, so finally got it on a Windows tablet and upgrades went fine from there. I could not get Homeseer to work as a secondary controller to Vera so had to unpair each box from Vera, pair it with Homeseer, upgrade, and re-pair them again, going through scenes again and re-mapping them on ha-bridge. So it was a PITA, but worth it now have working dimmers.