People are already complaining about image size bloat, and tshark contributes a lot to it. Its installed size is 77Mbytes, second only to the kernel and the mellanox drivers (whether we can reduce the size of mellanox drivers is another question).
It depends on the same libraries as the big wireshark, but is obviously not capable of analysis and most other cool things we love wireshark for. On the routers, people normally just save a dump to a PCAP file and copy it to their workstation for analysis. While tshark offers more advanced filters and some nice options for VoIP, tcpdump is sufficient for the job.
When tshark is no longer required for any of the vyos packages, it will be safe to install it from Debian repos, if someone really needs it.