A victory in the West implies French capitulation and British withdrawal from the continent - i.e. a 1940 situation. Unlike 1940, Britain can negotiate with Germany and believe that Germany is doing so in good faith. Also unlike 1940, Russia is already in the war and not in a bad position BUT it's on seriously dodgy economic ground, and its ability to fight on this scale without receiving finance and materiele is going to be measured in just a matter of months, during which things will be worsening and it will become obvious to people that a crisis is coming.
The question of Britain is essential to the question of Russia - the latter is absolutely not fighting on if Britain decides to call it a day. Germany in 1914/15 won't have acquired the harsh war aims of later in the war - it will want the rest of Alsace-Lorraine and the demolition of French forts, it might want basing rights in Belgium, it might want to annex Luxembourg, and of course it wants reparations from France, some of which may be in kind.