Try the Esxi5 Driver Rollup 1 Cd/DVD. Boot this, apply it, and it will install v4.0.355.1. Maybe flash the firmware to the latest as well. I did both of these and it seems to have solved the problem. VM's now work on a vswitch with VLANs.
The Driver Rollup CD does not do any firmware update. HP should release the new firmware again.
The latetest HP exsi release (october) now contains the updated driver. The new firmware (4.0.360.7) is also available. For more information see: c03005737