He is a bit undersized, maybe 10 lbs for his height, however the real issue with Vilma as a 3-4 ILB is that he is a sweeper by nature. 3-4 ILB's are not sweepers, they're gap filling hard nosed players who bring an impact to the position. Zach Thomas is lighter than Vilma and shorter but he hits much harder. You have to hit hard to be a 3-4 ILB it's just part of the job description and Vilma does not do that and never has.
