The rank of "Centurion" covered a very wide range of equivalent modern officer positions. The lowest grade of centurions commanded a century of 80 men, analogous to a modern Captain, with his Optio in the role of a lieutenant. More senior Centurions also held command of their Cohort, a formation of about 500 men that would be led by a modern Major. The senior Centurion in the Legion was the Primus Pilus, who acted as an adviser to the Legate in command of the Legion as well as the voice/leader of all the Centurions. This was the highest rank for the professional roman soldier, since Legate and Tribune were political positions given to aspiring politicians and not earned though service in the legions. The primus pilus would be somewhat higher than a modern colonel in their responsibilities.

Vorenus first introduces himself as "second spear centurion" which I would assume the showmakers derived from Primus Pilus (first rank, with a possible mis-translate of Pilus into "spear"-Pilum), a "Secundus" rank below that. Soon after he is promoted to Primus Pilus, which shows he was a senior officer to begin with and then got the top spot. Roman army commanders were heavily reliant on their Primus Pilus in the same way that modern officers are heavily reliant on their senior enlisted; he is the guy with the most experience and his word carries weight in the ranks. It would not be surprising for a Primus Pilus to be in the inner circle, especially considering the fact that by the time Antony got to Alexandria, the rest of his advisers had already ditched him, leaving only Vorenus.

The implausible part is how young Vorenus was when he was promoted into Primus Pilus. The age of his wife and daughter suggests Vorenus is in his early thirties, which is far too young for such a senior rank. Alternatively, it can show that if Vorenus got the rank in his thirties, he was an exceptional officer and would be prized for his input.

Maximinius Thrax, a provincial low born thracian, rose through the ranks of the Danube legions and became emperor in 235 AD and there are several examples of low born "noboydy"'s rising in the legions.

See also Galerius. A thracian/dacian born goat herder who joined the legions, fought under emperors Aurelian and Probus and was made co-emperor by Diocletian along with Constantinus (Constantine The Great) in the Tetrarchy.

Sometimes reality is stranger than fiction.

In the Augustan period being a primus pilus qualified you to enter the equestrian class after you completed your service at that rank, which thereby qualified you to apply for a magistracy or serve in a more senior officer position, such as praefect, which only an equestrian could fill. It was in this position as an equestrian officer, a praefect, that he was in Antony's 'inner circle' in Alexandria. This was a reflection as much of his position in Antony's officer class (in reality there must have been higher ranked officers among Antony's Roman troops) as Antony's personal trust in him however.

This sort of rise in influence by a common soldier is not typical, but was possible in the Augustan period. In one case, a Cneius Pmpeius Proculus was "centurion of several legions, primuspilus of the Fourth Flavian Felix Legion, tribune of the first cohort, procurator* of Bithynia-Pontus", but this was not a common thing (Le Bohec 44-5). Social background was essential to one's progression through military ranks in this period, especially because they entered the army at a higher rank in the first place (44), with merit being secondary to this (45).

I am actually not sure however whether this represents a normal career progression in the Late Republic. In fact I think it's quite fictional and I can't think of any common soldiers getting into the Senate in this period, as did Vorenus after being given equestrian status by Caesar which allowed him to campaign for an office (as quaestor I assume), which automatically qualified him to enter the Senate. So I would say that this is not a plausible career progression in the period that HBO Rome represents.

*Procurator is basically treasurer for a province, and is sort of analogous to being second-in-command to the governor. A procurator could even command a smaller province entirely.

Source: Le Bohec, The Imperial Roman Army, 1993.