I think this is spectacularly hard to get a robust estimate of, but my wild uninformed guess is your chances of dying of it interacting with your heart condition are less than 25%, and probably less than 5%. (I try not to pull probabilities higher or lower than 5%/95% out of the air - I need a model for that.) That's for the simple case where you don't get addicted and take ever-higher doses or start taking other stimulants too or start smoking, etc.

The only hard information I can get a handle on is that the US manufacturer lists existing cardiovascular conditions as a potential contraindication. I suspect this is on general principles (stimulants are known to make them worse, modafinil is a stimulant, sort-of) rather than on hard data about problems caused.

Reporting systems for drug side effects are haphazard and leaky at best, and it's very hard to do decent analysis. Unusual combinations that aren't very deadly just aren't going to show up in the research.The fact that we haven't heard that it's deadly does, though, put something of a ceiling on just how bad it could be (my 25% above).

Most medics would reckon taking stimulants you don't have to when you have a known cardiovascular condition is unwise. (Although some of them do it themselves in early career.) Quantifying 'unwise' is tricky. There's the general issue of data I just mentioned. Then there's trying to think it through. On the plus side, modafinil is less likely to cause problems for CV patients in the way that other more general CNS stimulants are known to; but on the minus side, we don't properly understand how it does work.

Doctors are by nature very cautious: "first, do no harm" and all that. You might come to a different cost/benefit decision.

FWIW, I wouldn't take it in your shoes. But I don't take it myself, despite having no contraindications. I'm extremely risk averse, particularly about my own life, and place more emphasis on quantity than quality compared to most people (on hedonic adaptation grounds).