19 year old Sam wasn't on their radar at all back in time, but they both knew John Keel himself on the #1 Timeline and the Sam 'Duke' Keel from the #2 Timeline Vimes version of Keel. So there's no conflict at all with their memory of lilac time when #2 timeline kicks in and can see Vimes in his post time travel state (well Vetinari at least sees him immediately with the eye injury etc and so realises the #2 truth).
For the Watch members it's different and essentially it's their recall of young Sam for 30 odd years as Sam their comrade who's grown up with them (in both timelines of course - that doesn't alter in the slightest) and so, with the passage of #1 timeline (which is no different for them from #2 timeline) and that long association they've never put 2 and 2 together because Keel has always been the Keel in the graveyard from their memory of him and Sam's grown with them - they wouldn't even remember Keel that well in real happening time and have no reason to question their own eyes after such a long time or if they did would only think that he vaguely looks like Keel. Familiarity breeds expectations and they just wouldn't see it except as a strange co-incidence
