Such "reuse" flag should be available in context menu of any tab, but state of it should be exclusive. If you set it for one tab, it should be removed for previous one.
I also agree with Alan about if tab is modified, new tab should be open which will get "reuse" flag set automatically (this removes the flag from previous tab). Maybe this situation can be solved in a way that if your current reuse tab is modified, you will get a popup, asking you save, not save or cancel. Cancel in this way will lead to leaving of the current tab open and opening of the new "reuse" tab. But this is also not transparent, maybe new popup, with request to open a new one or saving and reusing current one will be better. And popup with "do not ask again", so this will be one time decision based on user preferences. More or less self explainable...
With such "reuse" feature, for me still not clear a scenario, than not a current tab used for reuse... This will mean that some open document, in tab which is invisible, will be silently replaced. I do not like the situation, that you do no see what is replaced...
But I would repeat, I think it is good in this case to have both as VS as this "reuse" tab way.