Yes it is valid for that, however as you have been informed doing so as you plan will raise all kinds of questions with the Policia Federal and likely cause your being denied entry to Brazil altogether. If you have any knowledge of Brazil and the Policia Federal you'd know they don't play games and they're not the kind of people you want to make suspicious in any way. They can make one's life a nightmare.
The VITRA is valid for travel only, it is not intended for tourism purposes and it is expected that one would be going directly to their ultimate destination. The VITRA is issued for 10 days which would allow you sufficient time to reach the Brazil/Chile border even by ground transportation (provided you did not make stop-overs). Since you'd have to leave the international transit area of the airport in order to go to an airline ticket counter to purchase the São Paulo - Ecuador ticket, you would have to pass through immigrations in order to do so. That's when all your problems will begin. Far better to have the ticket in hand upon arrival and not need to leave the transit area at all.
I really doubt that the Policia Federal are simply going to accept that you couldn't pre-purchase the passage from São Paulo to Ecuador in Dubai. It's a simple matter of going to any travel agent and saying you've got a ticket from Dubai to São Paulo, they will simply arrange the São Paulo - Ecuador leg of the trip. You could also probably do the same online by yourself if paying by credit card using a site like www.decolar.com or any of Brazil's national airlines websites too. At least if you do that in advance it's going to go a very long way towards eliminating any potential problems with immigrations.
Another point to consider is that your entry to Brazil will be registered in the immigrations computer system and your exit from the country either via air or land will also get entered into the same system. If that does not take place in the alotted period of time alarms begin to go off.
I really don't understand the resistance to pre-purchasing the São Paulo - Ecuador leg of the trip there in Dubai. If I don't understand it, imagine just how much more the Policia Federal won't understand it either. Even with a VITRA and an onward ticket to Ecuador the Policia Federal may still want to see a return ticket anyway. Certainly the Ecuador immigrations authorities will too. If you don't have one you will certainly be denied entry to Ecuador.
I must ask you, and I do so very seriously... Do you really want to risk getting put right back onto the plane you arrived from Dubai on? That is a very likely scenario. If that happens you have absolutely no option but to come up with the money to pay for that return flight right on the spot, because the airline isn't going to take you back to Dubai for free.
Cheers,
William James Woodward, Expat-blog Experts Team