I'm planning on replacing the carpet in my dining room with a vinyl sheet flooring. I know I need to get the carpet, padding and any staples/debris cleaned up before laying the vinyl, but do I need to do any other prep work on the plywood subfloor (ie. fill cracks between sheets of plywood, etc.)?
Vinyl isn't very thick, and it is quite plastic. This means that over time, it will conform to the shape of the floor beneath. This means that you will see grooves in the vinyl where there are gaps between the plywood sheets.
This means you want the sub-floor to be as smooth as possible. BUT!!!! you need gaps between the sheets to allow for expansion or else you risk having the sub-floor develop a slight buckle.
Here's what I would do under ideal circumstances:
However, you need to make the call as to the quality of the existing floor the level of imperfection you are willing to tolerate. It may not be worth the extra expense and labour to do it this way. In which case, I would fill the worst gaps, sand down the worst edges, and lay down the vinyl.
After making sure the surface is clear of all miscellaneous stuff (staples, nails, tacks, etc.), inspect for dry rot or weak areas—hammering the surface to test strength every foot or two would not be overkill. Replace any substandard panels.
Then use a leveling compound to "erase" any knots or gaps in the wood. If there are holes, plug them with a dowel or other similar means, sand smooth and (if needed) use leveling compound to smooth them out.
Also, stomp around on the subfloor to identify any loose edges and squeaks. If you find any, screw the areas securely to the floor joists. Beware of squeaks coming from the floor joists, though typically most are caused by the subfloor edges rubbing against itself or against the joists.