My straight forward advice is - you are not likely to have a problem when you come directly from the US. The worst thing they can do is have you take the vaccination at the airport (make sure they use a new needle or bring one with you.)

The longer answer is that 7 years ago i crossed the border from Brazil to Bolivia. While I knew that a yellow fever vaccination is required to enter bolivia from brazil, I did not take it too seriously (or thought that the Bolivians won't take it seriously...) As I reached the border I realized that I was wrong - the bolivians sent back everyone with the vaccinations. It wasa real middle of nowhere crossing and I was desperate not to stay there for another day so I took a pen and handwrote on my vaccination card "Yellow Fever Vaccine, Name of city and date". The immigration officer took a look, tried to trick me and asked me where I got the vaccination (LOL) I told him the name of the city and he let me pass through. Funny enough, there were 6 guys behind me (same nationality as myself) that really took the vaccination but their card didn't say Yellow fever vaccine, but the name of the active material in the vaccine. The Bolivian that didn't know what it was didn't let them through and sent them back to Brazil to get a vaccine.