If they can make it to bases that are yeeeeaaarrsss away, they can blast out of orbit of one freakin' planet and escape its star system. If Delta Vega would trap them like that, then it's going to be the skeleton of Kirk's great-great-great-...-great-granddaughter who makes it to the nearest starbase.
A couple of things:
If low orbit can slow a ship at warp speed down to a few hundred km/s why couldn't the weaker impulse engines also have a hard time powering away requiring more power to fight the gravity or magnetic pull than flying in deep space.
They were in a hurry to get rid of Mitchell - perhaps they needed to expend more fuel than intended to accelerate and decelerate to Delta Vega in as short a period of time instead of taking a more economical acceleration profile.