It surely was forced to land... somewhere. However, this plane is capable of crossing the Atlantic, and surely wasn't going to make unnecessary refueling stops. And indeed, they didn't make the implausible claim that they'd run out of fuel; they made the only slightly less implausible claim that they weren't sure of the fuel situation. And several European countries dispute the chain of events. The plane in question was a dassault falcon 900ex - https://www.flightradar24.com/data/aircraft/fab-001 - that has a range of 8,340km; it would have needed only 1 refueling stop between moscow and la paz. The fact that portugal refused a refueling stop is perhaps relevant, because that stop may well have been planned for the azores - those are slightly further from Vienna than Moscow is; making the story about fuel yet more implausible.

Finally, consider the motivations of its owner - Evo Morales. While I have no objection to his political aims; it is nevertheless wise to consider that he does have political aims. And as it so happens, he's a populist, anti-imperialist, anti-US political leader. For him, seeking this confrontation would have been a pure win; and furthermore being seen as the victim of imperialist oppression would have been the cherry on top. As such, it's just not reasonable to assume that his actions were aimed to defuse the situation; if he could appear to be victimized by imperialist forces, that's to his benefit.

So it's entirely plausible for him to choose to land in Vienna instead of return. It's not very plausible fuel was running low. Additionally, there are about a dozen other countries very close by he could like have chosen to land in, even if fuel was scarce - he wasn't in some inescapable box.