I doubt LVG will be foolish enough to sign Falcao.
LVG has already stated that he is looking for a 20 goal/season striker. Falcao has scored 4 goals this season (which is extremely low for a striker), so it will take a huge leap of faith to believe that Falcao will score 20 goals next season.
And dont forget that Falcao had a disasterous game when he was played in the U21 game, where the U21 manager took him off for being crap. That manager would've given LVG a detailed report of just how bad Falcao is.
When you consider that LVG's job could be on the line if he signs a lemon - I doubt he will risk it.
So, why is LVG giving Falcao cameo roles? Just in case Rooney, RVP AND Wilson are all injured, they need a striker to immediately take Rooney's place. My understanding is that RVP and Wilson are injured, meaning we are just 1 bad tackle (on Rooney) away, from being forced to play Falcao as our lone striker.
...and with regards to the theory of "a deal was done in principle last Summer" - I doubt this is the case, but even if it were the case Man Utd will probably pay an undisclosed compensation fee to Monaco and tell them to keep the player and cancel the agreement. Whatever happens, unless hell freezes over, Falcao wont be a in a UTD shirt next year.