Could heavy straining like heavy sneezing coughing and vomiting etc cause blood vessels in the iris to burst/break? if so what are the symptoms?

No: We don't see blood vessels in the iris break in these scenarios, however it is very common to get subconjunctival hemorrhages where a blood vessel on the white of your eye breaks, causing an appearance of a red eye.

Yes: Yes it can , it called subconjunctival hemorrhage, there are no symptoms except red non painful but badly red looking eye without any change in vision.