Useless bit of knowledge of the day:
After debugging why an IP camera video stream does not work for a VPN client, I found out that VLC supports an additional option that enforces TCP transport for RTSP streams. Just add ":rtsp-tcp" to the extended options of the stream.
(From packet captures it seems that UDP streaming suffered from an MTU problem and the VPN dropped most of the RTSP frames. There's no option to limit UDP frame size. For TCP, either MSS clamping or PMTUD take care of that.)
I totally did not spend all of the afternoon on that problem.