The 12th Vegan Rangers has been described as the personal army of the FedSuns' Duke of Verde, which he converted to mercenary service during the Succession Wars. FM: Mercenaries says that they fought for the Federated Suns and Lyran Commonwealth to bring in money for Verde's economy, while the original "Galtor Campaign" noted that they also fought for "Princes of the Periphery."
Looking through the "Fox's Teeth" scenario pack, the opening scenario ("Cry Vengeance!") pits the titular unit (part of the 7th Crucis Lancers) against elements of the 12th Vegan Rangers during a DCMS invasion of Harrow's Sun. Even if the FedSuns Duke of Verde was using the Rangers to pad his personal finances, it seems doubtful that he'd hire them out to the Draconis Combine for the purpose of invading the Federated Suns. (At least not without facing treason charges.)
So, was the Duke of Verde a traitorous so-and-so circa 3020, or would it make more sense for the OpFor to be the Combine's Legion of Vega, rather than the Vegan Rangers?