Not entirely sure why people are jumping to conclusions about practice speed. I'd suggest going and checking how far behind Vettel Hamilton has been in p2/3 in the past few races, the answer is 0.6-1.4 seconds behind in pace.
Particularly at different races different cars are practicing different things. RBR need to work on the car with the highest top speed possible so that is their focus in every practice, more so at these tracks. Merc has high top speed and all their practice is focused getting life in tyres, reducing temp and doing race runs, they aren't focused on top speed. Despite being 1.4seconds behind Vettel 2 races ago in the same practice and 0.8 I think last race, he still got pole.
Merc has that speed on tap, practice isn't an indication of RBR dominating speed/quali/practice. As plenty of pundits said, Belgium had a corner that only the RBR could do without trouble and then use a great exit to get past people on that straight. At Monza there is less "good" corners for RBR into the long straights and more ability for the top speed to equalise against the RBR. That doesn't mean Hamilton will get Pole, but it should be fairly obvious that when they want to turn up the engine and bone everyone including Vettel in qualifying Merc has done so in every race outside of the first 2-3 races.
Overall I'd say on those gaps Ferrari and Lotus are closer than they usually are at this stage of the weekend, and we already know the Merc can gain that time when it comes to Q3 pace.
Also the important thing is, a car over a clean lap is faster with more downforce here and Spa, but less downforce is what actually lets you pass people on the long fast straights. If Hamilton gets pole, with no Eau rouge to help them into a straight, Red Bull are going to struggle to pass cars and Ferrari/Merc and possible a couple others cars have a very real chance of passing them with the higher speed.
RBR were 7th fastest in the speed trap at the end of the straight in Spa, but 1st at the beginning of the straight.
The difficulty is Merc not being great in the first couple laps, Red Bull being okay and Ferrari being often great, without Eau Rouge to help him, in MOST races recently the Merc's have managed to hold a lead up to the first round of pitstops. If that happens again then Vettel could easily come under a serious challenge from Ferrari/Lotus's behind them.
I don't think Hamilton will win, its possible, more so if he gets help and people held up like hsi last win. But I think if Vettel can't pass Hamilton very quickly the race might start to favour Alonso significantly. Alonso to pass Vettel in the first stint and get Hamilton after the first pit stop sometime.