I better start with stating my viewpoint very clearly: It's complicated.
The geographical differenses between between the east and the west was a large factor, which changed over time. The different actions of the western airforces had different impact at different times. Until late 1941 RAF was the only one western airforce that was directly involved, and the USA didn't do a lot of fighting over Europe for some time to come after that. And by the way, Britain and the Commonwealth benefitted from lend-lease too.
In the darkest period for the USSR, say until the onset of winter in 1942, most of the Luftwaffe was consentrated in the east, thereby cutting some slack for great britain and the front in North Africa. Though by no means intended as such by the USSR, the very existence of the eastern fronted helped. One example is the air supply in the winter of 41-42 at Kholm and Demyansk. It tied down germen supply aircraft and used up fuel that could have been used elsewhere. While I am not in any way certain that Germany could have won the war by applying more pressure on the British Empire, principally in North Africa and the Mediterranian, they could have made the going a lot harder. In this critical period the largest contribution of the VVS probably was that it refused to die. One undeniable achievement of the Soviet system was the willingness and ability to mobilise new forces to replace the immense losses all arms suffered in the beginning. Even though it got better, the strain remained throughout the war. In the process, Luftwaffe never got the chance to either consentrate elsewhere or take a break and really expand to a bigger force than the one it started the war with. Of course eventually the bulk of what was in the east, and southern Europe, had to be redeployed to the defence of the Reich, but the eastern front was more than two years old by then. Lendlease had comparatively little impact at the very beginning, but gained in importance throughout. It contributed more to the victories than to the bare survival, though it was never inconsequential.
Until the summer of 1944 the air war in the east and the west differed immensely. Southern sideshows apart, it was in the west the defence of one Island with an extensive and modern chain of radar stations and a defence system honed to perfection in the battle of Britain. The coastline, even where the continent was close, at least protected against the radar stations and forward airfields being overrun by panzers. That went both ways though Germany had a far longer coastline to defend. How good their defence in depth had to be changed as RAF got better at bombing the right country by night. In numbers and precision they didn't really start to matter until well into 1942. Effects on german production were neglegible at least untill that time too. In comparison the eastern front stretched from the arctic to the Caucasus (in length of the line itself it was at its biggest in the autumn of 1942), and constantly threatened on the ground. The corresponding air campaigns lasted without any real letup for nearly 4 years. There were blizzards and periods of less activity. Even after the (proper) second front opened up, it never stretched as far as the eastern, barring perhaps the last weeks. In the West Germany eventually faced not one but two major airforces, in the east the VVS faced Luftwaffe alone, together with miscellaneous German allies of widely different capability. (Yes, the two airforces did help by distracting the Luftwaffe, but it helps when one can specialize in night bombing and interception, while the other can specialize on daylight bombing and long range escort.) That also fluctuated over time until eventually none were left, possibly excepting the Slovak airforce.
I am NOT denying that the VVS and USSR got a lot of help from the western allies in different ways. Even then it is a remarkable feat that the VVS continued fighting in very adverse conditions. The precise weight of this and that contribution at certain moments cannot, in my opinion, be measured with much exactitude. Still i do enjoy threads like this very much, as especially the war in the east still present us with many riddles. The shortcomings of the soviet system and the animosity between the erstwhile allies are very much contributing factors for this. They even overlay the usual national national biases that also becomes clear when the question arises who contributed even more than the others to the defeat of the axis. Without answering the question exactly how good the soviet airforce was, I will venture that in relative terms the soviet army made a greater contribution to winning the war.
This is a long post on a rather generalized level. Luckily I am in no doubt that the following posts will go into details. I await in eager anticipation.
