Hello: We took a Viking River Cruise from Moscow to St.Petersburg in 2002. Most women wore capris or slacks for day tours. For evening, we wore slacks and pretty tops. For the Captain's reception, I brought along a long skirt with a dressier top.
The ship is probably consistent with Vikings other Russian ships -- not elegant, but comfortable, with good food. Have a good time!
I am very interested in this itinerary, so please post a review when you get back.