Hi,
They are possibilities but if you really want to force a vendor ; best would be to use a custom plugin to force a filter on a specific vendor.
Thanks to that, you could open the iframe on a specific domain (or using a parameter that you'll store in the session) and it would do the trick.
(you can find some filtering plugins in HikaShop and in HikaMarket, like the "vendor location" one).
About the payment, I just want to warn you that paypal can't be display in an iframe.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.