Those crew that you saw on the 763 should have been wearing their vests or jackets and if not should've been told to put their vest or jacket back on, but sometimes the CSM (manager) is just as bad as the slobby crew. There are a lot of people out there who go against what the "rules" are purely just to go against the rules. There are also a lot of people which are "thermally challenged" and are allowed to work in the shirt only, but only on presentation of a doctors certificate confirming this.
Basically, you HAVE to wear either the vest or jacket during the service, just the shirt is not good enough. On international sectors you have to wear either the jacket or after takeoff change into the serving jacket and then back into the normal jacket for landing. I think in summer, domestically you don't have to wear the jacket all of the time etc, vest only is fine.
Just something to think about. Next duty have a look at the people you're working with, hopefully most have made the effort to look like we're supposed to, something of which you sign as part of your contract. If everyone's spot on with their uniform and grooming and yet one person isn't, that one person brings down the look of the whole crew. Don't be that one person. It's really not that hard.
Oz