What's the correct way to say, according to the phraseology;
I will leave the runway; I will vacate the runway; or I will exit the runway?
The correct term is to vacate a runway. The term is used several times in the ICAO Manual of Radiotelephony (Doc 9432), including the following example (among many):
4.9 After landing
Unless absolutely necessary, controllers should not give taxi instructions to pilots until the landing roll is completed. Unless otherwise advised, pilots should remain on tower frequency until the runway is vacated.
Pilot: Georgetown Ground Fastair 345 runway vacated
ATC: Fastair 345 taxi to stand 27 via taxiway alpha
And a relevant note from the same document:
Note.— The runway is vacated when the entire aircraft is beyond the relevant runway-holding position.
I've never heard the term 'vacate' for this, though there's no reason they cannot do so. Most controllers will say 'Exit runway when able' or at a specific intersection 'Exit at Alpha two', etc. as a pilot, I have always said that I am 'clear of the active' followed by my taxi request.