After leveling up a powertech i am giving the vanguard a try. First thing I noticed is that the vanguard doesn´t have the equivalent in elemental damage, i.e. some of the abilities deal kinetic damage, while the powertech equivalents (death from above,flame thrower) and others like flame sweep..., deal fire/elemental damage.
Considering that some of the basic choices in terms of threat generation for the tank build include the abilities that increase elemental damage and elemental crit damage - i'd say the vanguard has a clear disadvantage with regard to the powertech in this area.