I think no.
A piece is captured if it is trapped between:
d) an enemy viking and an enemy tower.
Figs. 1,2: the fixed supporting piece could be either a tower or a viking.
So, the tower must be previously present and cannot be the "active" part of the capture.(Bold and underline to add emphasis)
After thinking about it thematically in this very abstract game, I agree with you and think it should probably be illegal. Still I can't agree with you that the rules support it being illegal, it is to ambiguous for me.
I can't see from what is written where that situation clashes with the rules. I don't see how the tower must be previously present. It just reads that the fixed supporting piece can be either tower or a Viking, which it is, its a Viking.
Any rules lawyers about?