Hi Alan (and Vixen),
If I were to read between the lines, it appears that because of your past history together and the strong emotions involved in reuniting, it seems that the strength of the connection between the two of you is stronger than with you and Red. However, I also get the fact that you have developing feelings for Red as well that may or may not deepen over time.
As others have said, it does not seem fair to Red to intentionally put her into an "interim" role until the time that a deeper relationship becomes possible with Vixen. If you love Vixen and also have feelings for Red, then the sooner it is possible for everyone to talk about their needs and desires and what they are willing to accept and not accept the better. I know this is probably difficult, but at least everyone can make an informed decision about how each wants to move forward.
This kind of openness is risky business! I honestly hope that you and Vixen do get to be together one day as it seems like your connection has withstood the test of time! However, I also hope this connection does not come at the expense of who you are, and also at the expense of using Red as an interim until this is possible. All the best!