Perhaps there is no much difference when people use the term Sat/vbyte and Sat/byte they both in this current day and time refer to same thing except someone is being specific to a transaction initiated before the introduction of Segwit. It’s prior Segwit that Sat/byte was used, but now the introduction of Segwit only added the term virtual (v) but just people use the two terms interchangeably. How the difference is not really much and since transactions this days are mostly Segwit when some say the fee is calculated in sat/bytes it’s left for one to know that they basically mean sat/vbytes just not clearly stated. In reality the calculation always end up with same result which formula is total fee (in satoshis) = Fee rates (whether sat/vbyte or sat/bytes) just that in Segwit TX discount may apply and in Non-Segwit it doesn’t apply discount. I might wrong though, just what I’ve read before and I believe it’s correct