There was a thread already regarding this question :

en.community.dell.com/support-forums/network-switches/f/866/t/19439513

You can try enabling the jumbo frames globaly or the on each port not per vlan.

Example The following example of the mtu command increases maximum packet size to 9216 bytes. console(config-if-1/g5)#mtu 9216 Taken from this documentation, page 294. support.dell.com/.../cli_en.pdf

What is the reason that I can't enable jumbo frame per vlan?

If I can't enable jumbo frame per vlan, what is the command "ip mtu" for?

Maybe there was some limitations on old f/w versions,try updating to latest f/w

dell.com/support/home/us/en/19/Drivers/DriversDetails?driverId=TWGNF&fileId=3383798720&osCode=NAA&productCode=powerconnect-6224&languageCode=EN&categoryId=NI

in release notes I can see some changes regarding mtu.. and here I think is the answer :

ip mtu Command Changed (Maximum Value Increased)

The maximum value for the ip mtu command has increased from 1500 to 9202. If not configured, the IP

MTU tracks the interface MTU.

so go for an update, probably that should help..