I don't find sherry vinegar subtle or sweet. To me it is really distinctive and assertive. There isn't really a replacement, but any vinegar will do instead of none in a pinch and you have some good suggestions above.

yu-uhhhmmmm you should upload this recipe on your profile. and i like latoscana's thoughts on the subject.

One of the wine vinegars would be nice, or you could use lemon or lime juice in combination with the citrus in the salad. I make a lot of fennel and orange salad, and always use citrus juice in the dressing.

I would think a combination of red wine and pomegranate might be good with fennel and citrus (which sounds great, BTW). Sherry has such a deep, grape flavor, even in vinegar, and I wonder if pomegranate would also echo those fruit tones.

The mustard is such a strong taste, I doubt that the subtle nature of the sherry vinegar would be detectable.

I second that, but go easy on the balsamic. Just a drizzle should be enough, not a 50/50 mix with the wine vinegar.

I'd go with a mix of balsamic and one of the wine vinegars - which ever tastes best to you. It won't be exactly like the sherry vinegar, but a rough approximation and should work. (I seriously just tasted vinegar combos vs. sherry vinegar to try to figure this one out.)