Vincent's last ability doesn't care what Vincent's power is when it deals combat damage to an opponent; it only matters what its power is when the ability resolves. For example, if Vincent's power was 7 when it dealt combat damage to an opponent but is reduced to 6 just before its last ability resolves, it won't deal any additional damage when the last ability resolves.

If Vincent isn't on the battlefield when its last ability resolves, use its power as it last existed on the battlefield to determine whether it deals damage to each other opponent.

The damage dealt by Vincent as a result of its last ability is not combat damage and therefore doesn't cause the ability to trigger again.