Bingowings said:
^I really don't think Palpatine would replace fleets of ships just to avoid upsetting Vader in fact I think he would get a few chuckles out of rubbing salt into those old wounds at every possible situation to keep the guy burning with rage.
Realistically the ships would have just become obsolete over time as new ships replaced them.
Even over the course of the OT we saw the Empire adding new fighters and making small adjustments to their capital ships.
If the Rebels hadn't stopped them they'd probably have a Death Star for each province and expanded the Empire out further.
It was just one explanation. I also mentioned before that the old Republic vessels were remodeled into the Imperial Stardestroyers to remove any remnants of the old Republic.