Why would increasing the VLAN MTU size cause things to break?
We have a customer who wants 2000 byte packets for some reason, so I increased all the physical links to 9216, which didn't cause any problems, but it didn't actually allow the 2000 byte packets because you also have to increase the mtu on the vlans involved. On one of the intermediate routers (a Juniper EX4200), I increased the vlan mtu to 9150 (a convenient number smaller than 9216 for vlan overhead) and a bunch of other connections through the router broke. I could see making an MTU smaller breaking things, but not making it bigger...