I agree with you on the notion that Griezemann would be a vanity buy and we don't really need to buy a second striker, but wouldn't he be the ideal replacement for Wayne Rooney? The way we play now sees Pogba play as a No 10 or as part of a midfield 3. Griezemann's supposed best position would be as a No 10. Assuming Mourinho's long term plan is to have Rashford as the central striker, would mean, our front 6 would look something like this;
CDM - Pogba
Mkhitaryan - Griezemann - Martial
Rahsford
I do agree that he's not what we need as a midfielder is top priority especially if Carrick calls it his time and Schmidfield leave as well. Getting a player to alternate with Rashford is also key like you highlighted, but considering Rashford has mentioned that he would like to play as a No 10 in the future means, Griezemann and him can do a lot of swapping around. The movement the front 4 will offer will be just ridiculously delectable and with Pogba and hopefully a good Carrick replacement, there will be passers in the team who can pick up this movement.
As you can easily see, I am personally torn on this one.