Is it common to have a vasovagal reaction ofter urethral swab on 21 yr. old male?

Yes, teens and young adults are prone to vasovagal reactions (common fainting spells). A urethral swab is just the kind of thing that is likely to trigger it.

Unless this was done for legal reasons, where cultures are still demanded by the courts, a DNA test can be done on a urine sample that is more accurate and does not involve sticking anything in anything else.