Prep one session around your players
After a few sessions of LMoP, you will have a good idea of how far your party can get in a day. Even though CoS is somewhat sandboxy, there’s no way they’re going to end up in
Vallaki, Kresk, The Winery, the Amber Temple, or the castle
in the first session. Travel takes time as well, and you can throw some combat encounters at them along the way to cities that you haven’t fully prepped to finish out the rest of a session. Both inside and outside the cities, there’s a hierarchy of things that do not need to be fully ready to go as soon as the characters reach the city.
Choose your session end point carefully
If your players are in a particular city with multiple things to do and see, and they choose a path you didn’t anticipate, leave them on a cliffhanger. Obviously this doesn’t work if you literally just sat down, but if you choose your end point such that a particular action on their part is unresolved, you know what they are going to do first next time. End the session when they, exhausted after a rough day of travel, finally see the gates of the next location. You then know that they will need an intro to a particular city at the start of the next session.
Many other DM’s have run CoS and written it up, on blogs of their own, reddit, and here. They’ve all fleshed various things out, emphasized the bits they or their groups found important or compelling, and can help direct your prep a little.
Withhold information from your players
Don’t give the players a reason to leave
Vallaki or Kresk
before you’ve fully prepared the next area. You should know where they are likely to go next, based on the hooks you’ve provided, and maybe all their investigative efforts until you’ve prepped what comes next point to things inside the city, or the questgivers could indicate that what they are asking for is a very dangerous endeavor. Or,
the Baron catches them on the way out and directly asks them to participate in that days festival, and we all know how much the Baron loves festivals. I had a festival ready to go that required the players participation and had to use it to keep them in town for the remainder of the session.
You’re the DM. There are so many tools at your disposal to keep them away from parts of the adventure you’re not ready for them to experience. Remember,
Strahd has his eyes and ears everywhere, and knows the players movements even before they do sometimes. If they try to go to the castle, Strahd could send them a message, or ambush them. The woods are fraught with peril and the Vistani and wolves are under Strahd’s influence.