Awfully odd way to structure this.
Does the ASA have an unused Ethernet port on it? If yes, just connect it to an access port in vlan54.
If no, consider trunking a port between the switch and the asa and create a sub-interface for vlan54.
This way all traffic restrictions are at the asa proper and not on the switch. As long as the vlan54 interface of the asa has a lower security level than the inside, zero traffic will be allowed, but all vlan54 traffic will be allowed out. Setting it up for the vpn to another company is piece of cake.