Using Amex Companion Cert w/ VDB voucher

I did a search. These threads indicate without source/support that it is not possible to combine an Amex cert with a VDB voucher:

http://www.flyertalk.com/forum/delta...nion-cert.html
http://www.flyertalk.com/forum/delta...-vouchers.html

This (albeit cryptic) thread indicates that it is possible:
http://www.flyertalk.com/forum/delta...nion-cert.html

I have a $400 VDB voucher in my name. I have found T fares DCA-SLC for $448 each for dates I'd like to travel. My goal is to use someone else's "Amex $99 DL Companion Cert" so that I end up effectively paying $48 (my ticket) + $99 (+1's ticket) + fees. All of this could be paid with the cert holder's Amex.

I read the T&Cs at DL's site here (http://www.delta.com/planning_reserv...x.jsp#platinum), but I'm not sure whether this is for the certificate in question. The only relevant term I see is "Primary Ticket and Companion ticket must be purchased with your Delta SkyMiles Credit Card from American Express."

Is this a YMMV situation, or are there T&Cs that prohibit the use of a VDB voucher towards the primary passenger's ticket when the secondary passenger is on a companion fare?

Thanks.