The spec says:

The VS field encodes the status of the vector extension state, including the vector registers v0–v31 and the CSRs vcsr, vxrm, vxsat, vstart, vl, vtype, and vlenb.

However currently VS is only dirtied when you write to vcsr. I think a call to dirty_v_context_if_present() is missing for most of the other CSRs.

Also I'm not sure about dirty_v_context_if_present() since it seem to be checking whether vector is supported which is always the case if we're modifying those CSRs so it's at best redundant, and at worst should it check misa[V] instead of sys_enable_vext()?