This movie seems to have an idea of impulse being really quite fast, such as when the Enterprise flies through the solar system and past Jupiter, personally I think this looks a bit too fast for impulse, and this makes me think that perhaps the Enterprise is moving at impulse through V'Ger.
The formula for warp speed mentioned above, suggests that when Kirk orders impulse engines "ahead ... warp point five", the ship should be traveling at 12.5% the speed of light. Covering 37,375 kilometers every second, I think the Enterprise
looked fine moving through the Jovian system. If I have time tonight, I'll try to recreate the scene with Celestia
and confirm or refute the shot.
Is there any dialogue suggesting they are at warp or impulse when they are in V'Ger?
I don't believe so, although the engines and navigational deflectors do provide non-dialog clues.