This is an end-to-end technology, so you would configure this on the SRC client/host. If you router has issues, disable MSS Clamping and set any MTU values properly. This will ensure the MSS size.
So just to be clear, this isn't possible. You may be able to alter the packet in transit (i.e. artificially altering the MSS Clamping), but this may cause other issues.
All VPNs would fragment a packet larger than the real MTU (provided the packet is actually that large), so it's not clear what your statement means.
(So then you disable MSS Clamping.)
- What's your real MTU!?
- How low do you think it needs to be?
- Why do you think that?
- Then why would you lower/alter the MSS?