Vanguard Veterans are almost a straight, though expensive, upgrade from basic Assault Marines Sporting higher, power melee damage from every weapon, they can tear heavy infantry apart, and go toe to with enemies that a normal Assault Marine squad would be crazy to drop in on. However, they lose Merciless Strike and access to any bombs from Thunder and Lightning, as well as any levels they had gained.
Keep in mind, you are paying to lose a squad and gain another, so timing is crucial. Instead of reinforcing you Assault Marines in Tier 3, you can ignore their losses and switch them out for a Vanguard Squad, mitigating the cost. You can even do this in the middle of battle to 'reinforce' a squad the enemy might think is done for, but be very careful. The switch from regular Assault Marines to Veterans takes a while and they may end up behind enemy lines if your forces are forced to fall back.
Upgrading to a power fist brings about a net DPS loss against heavy infantry and removes the considerable disruption the thunder hammer brings to the field. Remember that 50 DPS won't be enough to take down any vehicle that's not snared or otherwise distracted. Eldar grav-tanks, Razorbacks, Chimeras or Trukks are all viable reasons for the power fist.