Consider, however, that it took a advanced communications and scanning station like Epsilon IX (and at close range too) to determine that there was something inside V'Ger and not just an external Energy Cloud ("I have a null reading at the centre of the Cloud, "Definatly something inside there, but all scans being reflected back")
The Klingons couldn't have possibly determined that there was something inside the cloud, we know that in those days Klingons stressed more importance on "Military" focus than Scientific and Exploration
The Enterprise couldn't penetrate the Cloud, they had to actually go inside to determine that it was more than Energy, heck Starfleet assumed it to be an Alien Ship that was generating the Energy Cloud until of course they realised that there was no crew, or life in the traditional sense, instead they found a sentient machine that somehow aquired the ability to project an Energy Cloud of 12th power energy
Back to the main point...
The Klingons should have known better, at that range they would have detected the power output from V'Ger and should have withdrawn, however we know Klingons aren't like that, they found something violating their territory and they responded the "Honourable Way", by engaging the intruder and opening fire, although it was a complete mismatch, to retreat would have been dishounarable