Personally, I don't feel he was here long enough to leave a legacy behind, but the foundation he left is mostly having to be torn up and rebuilt by JM. In my opinion, LVG had our players scared to go forward, scared to make a single mistake, scared to play a more free-flowing, attacking style of football. JM is having to reaffirm and reassure them that it's okay to make mistakes as long as you are constantly learning and improving from them. He is having to re-teach them that you are allowed to pass the ball forward on the ground, instead of sideways, backwards, or punted 50 yards forward to Fellaini.
Also, I don't believe many of his signings will be around in 3-5 years. Shaw and Martial are probably the only "sure" bets there. I think TFM loves this club enough to possibly be a 1 senior club type of guy. He has forsaken international duty to focus on club duties. At his age, to ignore an international call up, that's a big statement!
As for the youth squad, I am grateful that he gave some players a chance. In turn, he also really screwed up Andreas Pereira's (my favorite youth squad player) and possibly Adnan's progression. Two years in a row he told Andreas that he was going to be used in the squad and therefore couldn't be loaned, and he wasn't used hardly at all. As for Adnan, he was loaned to a team that didn't even need him in the squad (Hence why JM was cracking some sniper shots at LVG for that). For the most part, I believe that those in the youth squad with the quality to earn first team debuts will earn a debut regardless of the manager, those on the borderline will probably be loaned out, and the others will be shipped on. While LVG might have had a slightly better track record at promoting youth than JM, I believe that quite a few of our lads would have earned debuts this year under JM if they hadn't already under LVG.
All in all, I'm glad he's gone, and things are already looking SO much better with JM at the helm. (This is coming from someone who despised JM in the past)