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Unfortunately you can't really rely on PMTU discovery these days. Not only is it regularly filtered, it slows every connection down since you add a RTT for the ICMP packets for every TCP host you talk to. In this case I would say the VPN client would be at fault for not adjusting the TCP MSS.

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Relying on MSS to not break layer 3 is going to be interesting when QUIC is more prevalent. PMTUD should be better supported.

QUIC unfortunately defaults to 1370 bytes to avoid having to deal with PMTUD :(. Endpoints are free to negotiate higher and the protocol does provide a way to generate arbitrary sized ping frames, but it's likely most implementations will just stick with the default.

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