You've made it quite clear that you don't much care for Rossi, but I find much of your criticism to be unfair or unfounded.
He is not in his 4th or 5th year of GP2. His first year was 2013 and he missed the first race. He also had several retirements. And he still finished 9th and well ahead of his team mate.
In 2014, he switched teams after the first 5 races and then only ran one single race before becoming the Marussia reserve driver. And it should be pretty clear if you watched the 2014 season that the Carterham GP2 squad were going nowhere.
2015 is or will be his first full season in GP2, where he's running a strong 2nd. And if you want to compare it that way, he's had just three more races in GP2 machinery than Vandoorne.
I don't think Rossi is on Vandoorne's level. That should be clear. But then again, nobody in the field is. I think Stoffel has the potential to be another Alonso or Hamilton. But I've been watching Rossi's carrier since he was in Formula BMW, I've seen him race in person more than once and it's obvious that he's no slouch and he has real potential.
I don't think he's ever going to be world champion. But he's quick enough and savvy enough that if he finds himself in the right car, like Jenson Button, that he'll be able to capitalize on the opportunity and show his mettle. To say he's a poor driver is really over the top.