It looks good. I.e., you are trapping the TAB in a specific tool state and preventing it from bubbling up. This would be a good coding pattern.
It is normal for TAB to move focus for GUI elements that have a TAB stop. However, Sketchup’s VCB does not normally need to have the input focus in order to accept values. (Meaning the user does not need to click on the VCB to give it focus.)
Do you see different behavior in older SketchUp versions?
BTW, please use the ```ruby block form delimeters for code when posting in the forum: