I agree that you should consut with a priest with more specificity.
However, you used two words here that jumped out — “affair” and “vows.” I think you already suspect the answer, and without condemnation, I read that the individual sounds quite involved in this emotional attachment.
I think he (you?) may be venturing down a dangerous path. The attention you give to this emotional affair will detract from your marriage no matter how much you think it will not. I do not believe it will stay the same. It will either progress or the two individuals will move on — but it will not remain static.
We will meet many “soulmates” during the course of our lives — people and relatives who just seem to “fit.” However, in using the term “affair”, I read that the individual is quite involved in this emotional attachment, possibly to the detriment of his marital vows. JMHO.