I couldn't resist vomiting, when I saw your fiancee for the first time.
The verb "resist" needs an object, as it is a transitive verb. For this reason, you have to use a noun. So, by using the gerund form of the verb "vomit," you give the verb "resist" it's required object, since gerunds are simply verbs used as nouns.
In the second part, it's more correct to say "saw" instead of "had seen," because you are talking about a single moment in the past, so the simply past form is sufficient. The past perfect tense (had + past participle) is only used when two different moments in the past are represented and the action that occurs further in the past uses the past perfect tense.