You wrote: “If my mom, a US citizen, was in England on vacation when I was born, would I be an English subject?”
Cruz’ Mom was not taking a vacation in Canada when Cruz was born. She had lived there for years; Cruz’ father was a Canadian citizen. She was listed on the Canadian voter rolls, which certainly suggests she was a citizen as well. If she registered Ted’s birth with the American consul, then Ted is a naturalized citizen, if she failed to do this, then Ted has a problem!
If you were born when your mom was vacationing abroad, and if she registered your birth with the American consul, the same applies to you. You would be a naturalized citizen, and you would have full rights as an American citizen, except that you could not be president.
This is Ted Cruz’ situation, IF his mother registered his birth. So far, Cruz has produced no paperwork regarding his citizenship, only a Canadian birth certificate.
Cruz’s mom lived in Canada for 3 years prior to his birth. At the time a person had to live in Canada for 5 years in order to gain citizenship.