I do think that 1977 is one of those years in which the player that had the best achievements and the player had produced the highest standard of tennis were different.
To me there's no doubt that Vilas had the best record, and so he was the player of the year and the real no. 1. However I think Borg produced the highest standard of tennis that year during his period of winning big titles on carpet, grass and clay. Borg's best tennis in 1977 was better than Vilas's best tennis in 1977.
Similarly in 1999, Agassi clearly had the best achievements and thus was the deserved year end no. 1 and player of the year. However the level of tennis that Sampras produced during his 24 match winning run over the summer when he straight setted Agassi 3 times, and during the YEC, was beyond anything that Agassi produced that year. Sampras's best tennis in 1999 was better than Agassi's best tennis in 1999.
1989 was a funny year because I do think that Becker had the best achievements and produced the best standard of tennis. He out-performed Lendl at 4 out of the 5 biggest tournaments (Roland Garros, Wimbledon, US Open and the Masters) and was the player of the year. However Lendl actually had a pretty healthy lead over Becker in the year end rankings. In fact some posters on this forum have shown that even under the ranking systems used in the 90s and 00s, Lendl would still have had a pretty clear lead over Becker points-wise, due to his excellent day-in day-out consistency that year.
Becker dominance in 89 far surpasses Vilas one in 77 since both won 2 biggies but Becker thumped Lendl twice there and Lendl was ATP 1