This is a physics question and so we let’s run some numbers.

We’ll use net change in velocity (not deceleration, because I don’t know how to determine the duration of the collision) of your van in this scenario as a measure of how fucked you are. Let’s say the bus weighs 45,000 lbs, GVW for a typical Greyhound bus. Your Ford E-Series cargo van will weigh 9,500 lbs fully loaded. This is not the typical passenger car scenario, but I’m going with the OP. If you hit the bus head-on on an undivided highway going, let’s say, 55 MPH, the momentum (which is a vector with magnitude *mv*) of the bus is 2,475,000 lbs-MPH in one direction, and your van has 522,500 lbs-MPH in the opposite direction. After a collision, due to conservation of momentum the fused mass now has a speed of 35.8 MPH in the same direction the bus was originally traveling. So your velocity has changed by -90.8 MPH in a very short period of time (oh, maybe a second or less). For your alternate scenario let’s say you have enough time to accelerate to 65 (pulling a number out of my ass) before impact. Now your momentum has increased to 617,500. The resulting speed of the fused mass is now 34.08 MPH. That’s slower than the first collision, but for you and your van represents a change in velocity of -99.08 MPH. So even though the net speed is slower after the collision, your increased speed just before the collision causes your net velocity change to be higher.

Braking before the collision allows you to stretch part of the deceleration over a relatively much longer period of time, reducing the forces at play when the collision happens. Hitting the gas is just suicide.