Vaginal ultrasound did not detect heartbeat at 6 weeks 4 days. Seen the sac and embryo. Risk of miscarriage?

50/50. At this point i would say your chances are about 50/50. A vaginal sonogram at 6 weeks and 4 days can usually see a heartbeat; but if your dates are off a little, it could be too early. As long as you aren't having a lot of pain or heavy bleeding, waiting a week and rechecking the sonogram is probably best.
Depends. If your dates are accurate your risk is high..However the most common reason fhr not seen is inaccurate dating.