I had a bit written up about this, but I felt it would intrude too far into the realm of telling the writer how to proceed, rather than how the character of Vimes would deal with the issue. As you noted, it is Vimes who would think she'd have a problem.
She could do more than watch...

Believe me, I don't see her as blind either. She loves Sam very much, and understands him as a husband as well as Vetinari understands him as a watchman. Her references to him as two different men, the husband and the watchman, imply that she'd be able to deal with the situation the way she deals with the bloodied clothes and the rumours of "late-night chases over rooftops." It seems very possible that she would understand, even facilitate the situation.

You know, I think you're right. I should've included this. Thanks!