During the years 78-81, the Aus Open had a very poor field. However, Mac did play the event in 83 and 85 for no result. Regardless, if he had played the Aus Open during his 'best of' years (not counting 83 and 85) and had won the event, I don't think anyone would have suggested it was a less than deserved win. As a result, neither Vilas nor Kriek (the former with a 5-6 record against Mac, the latter with a 5-12 record) deserve less credit for having won.
It is interesting to note that in McEnroe's 1983 Wimbledon win he beat Chris Lewis in the final, Lendl in the semi (remember, that was Lendl circa 83 on grass), Sandy Mayer, Bill Scanlon, Brad Gilbert, Florian Segarceanu and Drew Gitlin. Johan Kriek, in his 1981 Aus Open win beat Steve Denton, Mark Edmondson, Tim Mayotte, Chris Lewis, Tomas Hogstedt and Drew Gitlin (lost one of the rounds, sorry).
Now, player for player those two fields (not counting the winner) stack up well, except for Lendl. When it comes to grass-court tennis they're almost identical although you could argue that Denton, Mayotte and Edmondson were tougher opponents than Segarceanu, Gilbert and Mayer.
As I've mentioned before, Australian grass is wholly dissimilar to English grass. The bounce is higher and there isn't the same desperate need to play from the net (it is the percentage play but it isn't the only option). That higher bounce helped Vilas enormously although he did train like a spartan in order to make the adjustment. Kriek, similarly, had a better record at the Aus Open and US Open because of that feature.