Earlier quoted context omitted.
South America is not a US vassals. Some of the tiny countries that were invaded have strong dependency on the US. But Brazil, Argentina, Chile, Peru, Mexico, Venezuela I struggle to believe they are. Where ever did you get that idea from
Look at places holding US debt to identify vasal States. The debt is what defends the state. A coup or military takeover and that debt won't get repaid, so there is financial incentives for the US to intervene.
They pay the money upfront for them and if the regieme change was illegitimate enough to freeze the bond it would be "free money" until a legitimate claimant comes forward. Like a neighboring city's bank refusing to turn over money from the account of Gotham after joker took over the city. That is the only securing function of it - meaning others lose it if they steal it.