Let's remember here that this awesome 12th power energy field did zip in tactical terms. It was never said to endanger the ship of our heroes, or the ships of the Klingons. It was never said to block weapons fire. It wasn't even directly established as the reason one couldn't get clear readings of V'Ger itself.
It seems the energy field was either the unusually large starship's means of moving around at warp seven (and propulsive warp fields have never been weaponized in Trek, suggesting they are fundamentally harmless despite being impressive examples of space-bending), or then some sort of a more or less harmless byproduct of some other V'Ger function, such as its sampling of the universe around it.
Klingons would known when to cut to the chase. Pretty lights wouldn't distract them from confronting the enemy; indefinite sensor readings wouldn't make them hesitate with deciding that the enemy sat in the exact middle of its cowardly cloud.
And I'm not convinced the Klingons were moving at warp speed, we see the three cruisers slowly moving towards the camera in the opening shot, with the stars in the background.
When the Enterprise
hits warp seven after Spock's engine repairs, we see those exact same visuals: the ship moving slowly on the foreground, stars (rather than streaks) in the background. So we can't tell whether the Klingons were doing warp or not. FWIW, their engines are never shown glaring particularly brightly even when they are doing high warp (see e.g. "Flashback"), even if the star background effect in other movies and shows is less ambiguous on the issue of warp vs. impulse.