Tell the truth if you want. Or make things up. In fact, especially make things up.
Now, off to answer Londo as Vir. Because oh boy!
ETA: Okay, seriously? After writing this, I totally want to write a whole AU now with plot and everything, even though complete retrograde amnesia usually only happens in soap operas. Because not only would the Vir angst and the Londo&Vir be exquisite, but we would also have the whole matter of atoning for sins one doesn't remember.
*beats amnesia!Londo plot bunny over the head with a hammer*