Frustrated and low confidence

Hi all
I had the aortic valve replaced about 5 yrs ago and it was a total success.
I am now 78 years old and consider myself to be relatively fit,still working full time in the building trade.
About 12 mths ago was returning home from a job with my son on a very hot day and started to vomit seriously without any “evidence”.
I went unconscious for 20 mins until we reached hospital.was admitted
This situation has now happened 5 times resulting im hospitalisation each time.

symptoms are
serious vomiting,never any discharge.
dizziness and blackout
all color drains from face
very hot and sweaty
variation in blood pressure which stabilises fairly quickly

still no definite cause given by hospital but they think vagal prob.

Has dramatically affected my confidence and
reluctant to make holiday plans

questions

what causes the involuntary vomiting and why is nothing ejected.

is it possible to repair poss vagus damage.

any information gratefully received