You don't own a VECD license and unless you renew your software assurance each year you are illegally running VDI's. They've got you locked in. Have a gander at this from the FAQ on VECD
Can I continue to order or use VECD for Software Assurance after my agreement (and corresponding Software Assurance) has expired?
No. VECD for Software Assurance (the license intended for traditional PCs licensed for Windows Vista with Software Assurance) may not be ordered or used after expiration of Software Assurance coverage. The license is only available for PCs with active Windows Software Assurance coverage. VECD (for thin clients or Windows PCs without Software Assurance) may be procured under any valid EA, Select License, or CASA enrollment. However, the subscription co-terminates with the enrollment and may not be extended beyond the expiration date without the renewal of the agreement.
Shaking head, all these companies telling me that VDI is the end all be all here is another PITA that you have to deal with. I don't need software assurance. As a library we get software pretty inexpensively. We change our PC's/Servers every 3 years and get NEW OS's for them. There is no inherent reason to pay a yearly price for them.