What is the 'Interfacespeed' value which you are seeing the database in the interfaces table against this VLAN interface mentioned in the question.
Your switch itself will shows 1gb for VLAN's. It is basically the default. So if you did a SHO INT for any vlan interface it will show 1gb. I do not know for sure but its probably that way just to be uniform. That interface is a software SVI and there would be several limitations to its speed. For instance you may have a 1tb back plane but you might only be able to handle L3 at 100gb total for the chassis. So the hitch probably would be that all SVI's probably share the same pool of resources so assigning a bandwidth to one interface is somewhat useless.