Hmmm what part of DHS could need armored vehicles...
... oh... what about the division that has been operating them since the 40's; the US Secret Service?
The USSS is in charge of protecting not just POTUS and the VP but other heads of state on visits to the US and in some instances foreign embassies. The USSS also has several tactical response teams that might need a MRAP type vehicle to complete their mission.