It is true, ultimately no matter how good a driver you are if you haven't had any real traction or down force issues for the past 4-5 years then you will be more rusty at dealing with those issues than someone who has dealt with those issues for that length of time.
But a great driver should do pretty well anyway, however I said earlier in this thread, it will be interesting to see if Vettel can improve over say the next 2-3 months and several races. Can he improve and get used to these cars or will he not improve much, with the implication being that if Vettel is matching/bettering Ricciardo by the end of the season then maybe it does point to him being rusty and needing some time to adapt. if he remains not very good, it for me does rather back up the idea that he isn't that good of a driver. of course Riccy is a bit of an unknown quantity, it could be that Riccy is just about the best driver ever and Vettel is merely not quite THAT good.
I've never said Vettel is a poor driver, merely that I don't think titles proves he's brilliant. I think a season in which driver ability out on track has a huge impact, more than ever in tire life, overtaking, braking and harvesting. So far Vettel isn't adapting well, but could simply be more rusty at this type of driving.