Lol, 12m cash or Vucinic for Sissoko is indeed great business, but that's exactly why I don't think it will materialize. Roma aren't that stupid.
Vucinic and Melo have the same market price imo so a swap deal would be fair, but I'm not sure if we're improving the team that way since we're weak in midfield anyway and it will become even weaker midfield without Felipe, while if we sign Aguero our attack will be very solid even without Vucinic.
If the rumors about Roma selling De Rossi to Chelsea are true and if it's true that they want to replace him with Marchisio, then I'd rather we take it slower with Vucinic's transfer and wait for Roma to sell DDR first. When they do they will need a replacement and we can milk them. We can even ask 5 or 6m+Vucinic for Marchisio, which imo equals 20m eur.