Vilas getting the short end of the stick here. Vilas would have more FO titles (because we all know that would tip the scale in the minds of many posters), had he not run into prime Borg twice in the Finals. The loss to 17 y/o Wilander was shocking at the time...but being that Wilander immediately backed that up with great wins throughout '83 (including reaching the French Finals again and beating Lendl in Australia for the first of back to back titles), the loss to Wilander isn't like Coria losing to Gaudio or something
Federer and Djokovic both would have won MANY more RG titles without Nadal, and most people here, even Federer and Djokovic fans, do not give either virtually any extra unofficial credit for that. Now maybe you would disagree with that perspective, but that is the perspective most take here, with only marginal few dissenters. So naturally you shouldn't expect much to be given to Vilas for only losing out on 2 max RG titles to Borg, as that is the number of times he lost to Borg at RG. And anyway removing Borg at RG, particularly when factoring in the depleted RG field many of those as this was the period of the warring factions on tour leading to absences many times, Lendl's competition at RG and on clay in general is now far inferior to Borg.
Actually Lendl lost the 81 RG title to Borg in 5 sets, and almost for sure there was clearly nobody else beating him that year. Examining the draw, the form of the players, head to head of players who made it deep at the time, his toughest opponent would be Clerc, who he already beat in 5 sets in the semis (Lendl already straight setted McEnroe in the quarters that year btw, Vilas played that year and went out early). So removing Borg, Lendl still has more RG titles than Vilas, LOL! That hypothetical argument does not even work.
Another argument is who would they have beaten for these hypothetical titles, and who did they beat for their actual ones. Vilas beat Brian Gottfried for his lone RG title. Who does he beat for his possible other 2 without Borg? Well in 1978 the semi final losers to Borg and Vilas were Corrado Barazzutti and Dick Stockton. 1975 the semi final losers to Borg were Adriano Pannata (lost to Borg) and Eddie Dibbs. Actually considering Pannata won 2 of his 3 clay matches (their only matches on any surface) in 75-76, it is 50/50 at absolute best if Vilas even wins 75 without Borg. Panatta is also the man who beat Borg on route to the 76 RG title, he was a beast on clay around then. I am leaning to Vilas does not win 75 without Borg, but at best it is 50/50. So in fact removing Borg he might well be down 4 RG titles to 2 to Lendl, with a weaker field than Lendl now faced, particularly at RG to boot. Regardless the wins he has for his hypothetical 77 (well already real) and 78 titles are obviously extremely weak, with only 75, if he does win, being any quality opponent. Lendl by contrast beat peak McEnroe, even if on his worst surface, in 84. Beat Mats Wilander in 87. Beat Mikael Pernfors in 86, ok not that strong, but his wins in 84 and 87 were very strong. That is before even considering his hypothetical ones.
And if we are going to remove players might as well even remove Wilander, who is now FAR superior to anyone Vilas has left facing on clay, now that Borg is removed. Lendl obviously wins 85 now. Lendl lost to Wilander in 5 sets in the round of 16 in 82. Lendl and Vilas have a head to head history which opens with Vilas winning their first 3 matches, but then Lendl winning 10 of their remaining 12, starting in mid 1980. While it is possible Vilas somehow wins a likely hypothetical 82 RG final with Wilander, the more likely scenario clearly is Lendl winning that hypothetical final, even with his sketchy early history in major finals (not even meaning much in comparision to Vilas who is noted for his nerves in big matches and vs his main rivals too anyway), which would lead Lendl to 6 RG titles vs only 2 or 3 for Vilas in this new hypothetical. I also haven't gone into listing the players both actually lost to at RG, but while both had some bad losses, believe me this would also make Vilas look the worse of the two, even with his relatively shorter career. Yeah this argument doesn't work for Vilas at all. Lendl >>>>>>>>> Vilas at RG point blank, not even a hypothetical game helps Vilas here.
I am not surprised at all nearly everyone rates Lendl higher than Vilas on clay. The only argument I can see for Vilas is his large number of smaller tour clay titles, but Lendl has a very large number there too, even while less than Vilas. And that seems to come mainly down to willingness to play, Lendl was certainly no less dominant on clay in general, even outside of Borg.