Spilling into vanilla life- at work!

[chuck]

Well, when he said that he didn't tell it to me, personally, we were all talking in a group when he disclosed that information...

I'm curious. Was he the only male in that group?

I've told him I'm in a committed relationship (though I didn't feel it appropriate to say what kind), and he also is in a relationship (with a vanilla who clearly is not satisfying his needs).

This could be just an office flirtation with private fantasies, but you're playing with fire here. I suspect that you are an easy person to be infatuated with, particularly when that person has compatible character traits. If that happens, that may degrade his relationship with his significant other as well as be detrimental to the professionalism at work. Be careful. There is a high potential for non-fun hurt here.

It was suggested to me that I should tell my sub this was going on as a means of building trust and also dispelling the secrecy of it, but I'm uncertain. he doesn't have the strongest of self-esteem and I'm afraid he'll fear that is place as Mine is threatened (which it is not).

You are the best judge of his personality and your relationship. My uneducated guess is that being open and honest with him may help his self esteem (assuming you are not rubbing his nose in it). If he senses that he is being left out, he will feel more insecure.