I worked with another user who encountered this issue a few days ago. You're right, it's because our Remote Access feature is driven by tightVNC which doesn't support xrandr. Unless we change anything in that regard (it's not in our immediate plans) you have a few options as a workaround.
1) You can use one of the only browsers I've found that is stable through tightvnc, called hv3. See my post from that original thread:
There may be others of course. That browser did seem pretty bare bones but it behaved, at least.
2) Work around it by connecting to your Pi with some other remote desktop/vnc software outside of the Cayenne 'Remote Access' feature. Like the vnc4server that you've mentioned, or there are many other options in the Raspbian software repository.
I'm going to try to spend some time next week to figure out if there are configuration options that you can edit on tightvnc which will allow it to behave better in the size we open into via the Remote Access feature. We don't expose any settings for tightvnc through Cayenne, but I'm sure they are there in config files on the Pi, I'm just not sure which to change, yet. I'll update once I know more.