Wow, Chrissie, that's great! I was last year at a play, but down in the stalls so I'm not sure about the dress circle, but I'm sure it will be fine. It's such a lovely theatre, a wonderful venue for Rufus for sure!
Here you have the sitting plan and from what I remember the stalls where partly covered by the upper floor so you shouldn't be too far from the stage...
http://www.theambassadors.com/oldvic/info/seating_detail.html