Hi Peteallen, I can give you some things to start thinking about to achieve a seamless bow change. Of course, this has to do with tone production principles in general, which we should examine first. There are three basic aspects of bow function that effect tone. These can also effect the subtlety of the bow change.
First let’s create or select a simple bowing exercise that we can practice with. I always use what is called The Tonalization. I do a variation of this exercise, found in Book II of Suzuki Violin Method. It is as follows in the Key of G Major:
D3, D3, A1, A1 | A3, A3, E2, E2 | E3, E3, E4, E4 | E3, E3, E2, E2 | A3, A3, A4, A4 | A3, A3, A1, A1 | D3, D3
Play each note full bow, slowly, in half notes. The three aspects of tone should be considered as you play. The weight of the bow, the speed, and the placement point of the bow (where the hair touches the string)
In practicing this simple tonalization, make sure the weight of your bow remains the same at all points from frog to tip and in between. The same goes for the speed of the bow, which often changes dramatically just before the turn around at the tip. The point of contact between bow to string, often we think we can play anywhere between the bridge and the fingerboard but this is not the best idea. In truth we must look at the width of the bow hair. Then place the bow one or two ‘widths of the bow hair’ away from the bridge. This is your allowed contact with the string. From frog to tip, no matter what, your bow must not roam closer to the bridge or the fingerboard (unless you are doing it on purpose for a reason) in this exercise.
Using the vibrato to mask the bow change is something, to be honest, Ive never heard about. I think you can get virtually seamless bow changes without consideration of the left hand, if you have good bow control. Even if there is a ‘trick’ using vibrato, it probably assumes that you have full control of the weight, speed and contact point of the bow, keeping these three things always the same from frog to tip.
I hope this helps!