There is no question that the best way to warm up is to start with long steady slow notes drawn over the strings as evenly as possible. The question is how much time can we devote to warming up and what should we be doing at the same time? Should we just concentrate on tone production, intonation, volume and smoothness or should we even incorporate vibrato? I think that all of the above are productive. Begin with long, smooth, very soft or strong, even and clean scales and arpeggios; then add vibrato. Vibrating is among other things good for stretching and loosening the left hand fingers. Step up the speed progressively and increase vibrato speed and width as you begin to feel warm and loose. Take you precious warming up time seriously and enjoy a relaxed start. Make sure to start early to give yourself the time necessary for this important beginning.