The V'ger cloud is heading towards Earth at warp speed. In the very first shot of the film you see stars moving behind the cloud. It has to be at warp, anyway, cause otherwise it couldn't get from Klingon space to Earth in a matter of days.
There are consistency problems, however, as this isn't the case in subsequent shots of the cloud.
If the cloud is at warp, the Klingons must also be at warp in order to engage it, otherwise they're sitting still or cruising at impulse and allowing it to come towards them.
The VFX guys at Apogee, add to the confusion by having the impulse engines lit on the Klingon ships.
As I stated in an earlier post, on-screen evidence suggests that the Enterprise is at warp right up until the time it gets grabbed by the tractor beam. The nacelles are lit and the deflector is blue. The moment they "disengage all main drive systems" the nacelle glow vanishes and the deflector idles down to that amber color seen in all shots at sublight speed.
As to the 12th power energy field, I think the point is it's a gigantic/shocking/terrifying level of energy. Albeit not canon, Roddenberry's novelization indicates that the "cloud" is the glow of annihilated atoms hitting this powerfield. The cloud itself may be mostly harmless, but the very fact of its existence points out that whatever is making it is probably not something to be trifled with...going back to the "stupid Klingons" question.
Finally, once again to the Warp Point 5 question, here's what I said about it in another thread a while back
The evidence for warp .5 is right on the screen in TMP. Kirk's orders and Captain's logs make it plain.
Kirk's first Captain's log occurs after the ship flies by Jupiter and in it he says, "...1.8 hours from launch...", during which time the ship has been traveling warp .5.
At its farthest from Earth, Jupiter is about 3,220 light seconds away, or about 54 light minutes distance. At half light speed, it's 108 minutes away. 108 minutes = 1.8 hours. Ergo, if Jupiter were its maximum distance from Earth when the Enterprise launched, at half lightspeed the ship would get there in exactly the 1.8 hours Kirk cites, which is too on-the-nose to be an accident.
But, for the sake of argument... At closest approach, Jupiter is 1972 seconds/32.87 minutes away at light speed. To over that distance in 1.8 hours at warp .5 would put the ship at .30c (just shy of 1/3rd light speed).
So, is TMP is any guide, warp .5 is somewhere between 30% and 50% of light speed, with 50% being the more likely intention.