In my mind security comes first. In the vendor isn't up to required levels, I'd r-route the consumer but if they have to log in twice, they will get upset.
In the past it was a known problem to have a banks homepage spoofed with the online banking logon there, and the scammers would capture the credentials. I don't recall all the specifics but IT should be involved to avoid problems.
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AndyZ CRCM
My opinions are not necessarily my employers.
R+R-R=R+R
Rules and Regs minus Relationships equals Resentment and Rebellion. John Maxwell