I don't think it would have gone down too well with the fanbase, TMP being the first Star Trek feature film and all. I can imagine the 2nd movie would have served to bring Spock back and therefore TWOK would never have been made (at least not in the form it was made).
Nimoy was originally going to pass on "Phase 2" and then of course TMP came into fruition. Nimoy was enthusiastic about returning for the first live action movie (and also because they gave into some of his contract requests).
But hypothetically speaking, if Spock was to be killed off in the first movie, they only way I could see it fitting into the story would be Spock choosing to merge with V'Ger, instead of Decker. It makes sense in a dramatic basis, Spock was a kindred spirit to V'Ger, being logical, but yet trying to grasp the concept of his human side. However, unlike V'Ger he had the emotional capabilities and the potential to evolve. This is clearly stated in the movie as being V'Ger's desires, it had learnt all it could as a logical data collecting machine and longed (in a non emotional sense) to progress. V'Ger needed someone that would willingly merge with it and allow it to experience "Human" emotions and all that kind of stuff.