It's all an an increase in risk, but as long as you don't have any heart issues it should be OK. But it's not for anyone here to say definitely whether it's safe or not. MDMA increases blood pressure and heart rate, Viagra has a kind of opposite effect. There's limited data available for this combination obviously, but in your favour is the amount of people who try it with no known reported negative effects.
Since this site is concerned with harm reduction, then as has been said before, keep your dose small, like 25 - 50mg.
On a personal note, I've tried it and it works a treat, but it sure as hell gave me a headache and that can't be a good sign.