Gary, I don't think Vangelis' problem is with the actual TEMPO per se ... I think the problem is this:
Let's say he is playing a 4/4 style at TEMPO '90' and the vocalist holds a Whole note for 5 beats instead of 4 beats and comes in on the 2nd beat of the measure instead of the 1st ... the TEMPO stays at '90' but the vocalist is 1 beat off with the keyboard style ... Vangelis wants a quick and easy way to reset the style so that he is 'in sync' with the vocalist ...