Log into and use Plastiq in incognito. That should give you the vanilla Masterpass option.
If I’m not in incognito, Plastiq does something similar and only gives me the option to log into Masterpass using Citi Pay. Then I can’t access the MCGCs that are stored under my generic Masterpass account.
I’ve liquidated $1,400 of the MCGCs from Staples through Plastiq so far. It’s a pain activating each of them, registering them to your personal info, adding them to Masterpass, then adding them to Plastiq. But the card numbers are similar, so copy and paste helps, and you develop a rhythm. I had trouble activating one card, and I’m currently locked out for 24 hours.
Relatedly, I took
@SBWB48 ‘s suggestion and tried to add a Vanilla-branded VGC (bought at Walgreens using CF 5x) to Masterpass, then add the VGC via Masterpass to pay a bill through Plastiq, but got declined. It may work for Metabank VGCs (often found at grocery stores), but apparently not for Vanillas.