Have you used the virtual card with any other merchant? If it works that proves the number is valid.
According to this
site all 5403 numbers are Citi virtual accounts. I speculate that the merchant, or their card processor, knows this and they have made a conscious decision not to accept virtual account numbers.
Yes, they do work on other sites. I was thinking the same thing the merchant blocked it. But I was thinking maybe different cards with citi use different first 4 numbers instead of just 5403