As BobB mentioned, talk to the Contractor about the 'edges' that butt-up against things like windows, doors, 'vents', and other surfaces like brick facades, fireplace structures, and the like (IF you have them.) The final 'look' may not seem right. In other words, the finished job may look like the house was vinyl-sided over 'something else'.
Also, it may be worth reading the Warranty and Installation Instructions yourself, or at least contacting the MFGR. for these. They usually indicate 'surfaces' you can, and/or can't, vinyl-side over. Or, they tell you the 'type' of surface you can side over. Sometimes, the Installation Instructions will say, "... Install over a smooth, flat surface." Well, is cedar this type of surface??? Or, it may be written that, "... Furring strips need to be installed."
Do your homework BEFORE you buy/sign anything! The next thing you know, your shingles may start falling off behind your siding and causing a 'bulge' at the bottom. Or, you start seeing 'stains' on the vinyl (for some unknown reason.) I hope you understand my point.
Now, if I was you, but I'm not, but if I was, I'd remove the existing siding and install a 1/2" insulation board. Then, I'd tape the seems with something like Tyvek Tape. Then, you can caulk 'things' up while the wall is exposed and add new 'accessories' like Hose Bibs (spiggots) and Outdoor Electrical Outlets. If you're doing this job yourself, there's a whole lot more to know. If not, just talk to the Pro about it.