This is a good question. I would say do a little bit of both. It is nice to work on a specific technique doing scales, which vibrato can be considered as one of those techniques. I wouldn’t do it all the time though, as it is also good practice to perfect intonation by not doing vibrato on each note. Often times I do vibrato on the letter of the scale I’m doing only, which works quite well to get a little bit of both concepts in one scale. Nothing really changes for an audition in my opinion!