Twins/Risk of vanishing twins

Ok, so I had my first ultrasound at 7 weeks 4 days. And SURPRISE was told we were having twins. Baby A measured right on at 7 weeks 2 days with a heart beat of 155 but Baby B was only measuring 6 weeks but was able to find a heart beat of 129. Baby B and the gestational sac appeared significantly smaller. The ultrasound tech didn’t seem super excited or positive when measuring baby B but when meeting with the NP after she didn’t seem concerned at all. She said it’s good they both had strong heart beats but I am just concerned with one measuring 9 days behind. I know most like to be measuring within a week +/-. The NP didn’t ever really mention vanishing twin or suggest that even though we did talk about what to expect if I did lose one. Have a follow up ultrasound in 10 days to monitor their growth (Feb 26th).

Just want to hear your stories! Have you heard both heart beats and still had a vanishing twin? Has anyone had one measuring so far behind and catch up? I know vanishing twins is fairly common with multiples but just want to prepare myself!