Is (or was) it acceptable to throttle up for takeoff before being aligned with the runway center line?
Pilots in any size aircraft may sometimes be given an instruction from ATC such as "... Cleared for immediate takeoff ..." which means that the pilot is expected to execute the takeoff as quickly as he/she can safely do so.  That may involve applying noticeably more power than normal for the roll from the taxiway out to turn and line up with the centerline of the runway, to keep the speed up and get off the ground as soon as possible.  For example, there may have been an aircraft on final approach for the same runway and the air traffic controller wanted to be sure to maintain ample separation from the departing aircraft.  So unless you were in the cockpit hearing the radio communications and seeing what the pilot may have seen in the approach corridor, it's difficult to say for sure whether the pilot was hot-dogging or was just wasting no time getting out of the way of arriving traffic as soon as he could.

But on the other hand ... announcing to passengers "Nos vamos" while hitting the throttle in the taxi turn out to the runway centerline ... at best, that doesn't exactly sound like what a cautious and conscientious airline pilot would choose to be doing in that moment!

With two pilots in the cockpit it is not unusual for one to do the radio communications while the other manipulates the controls to fly the airplane.  Those words could have been quite fine for one pilot to say to another in the cockpit intercom as emphasis.  Maybe the pilot only meant to do that but mistakenly transmitted it to the passenger cabin instead; in which case that would have been an unfortunate and unsettling error although not an especially dangerous one.

Although I haven't done such a report myself (because thankfully I don't recall being involved in such an incident), if you observe an airline pilot doing something you consider very dangerous or inappropriate I'm pretty sure the airline would want to hear directly from you about it with details such as flight number, date and time of the incident, so they can investigate.  We can each do our part to keep flying safe!