"would be worth a lot of money...but then your daughter would lose a father instead of gaining a husband." Here we see inexperienced Michael violating one of Vito's cardinal rules: never make an unncessary threat. Later, the more experienced Michael doesn't rise to Moe Green's taunts and threats with his own--he knows what he has to do. Ditto Tom earlier with Woltz.
It was not a pure threat, it was actually a show of Sicilian sensibilities. Obviously Michael was an important person with body guards, etc. Vitelli was a small time restaurant owner whose daughter was of age to be married. Micahel was showing his respect for Vitelli by telling him he wanted to court and marry his daughter, thus ensuring her (they thought) a prosperous and long life. He certainly had more to offer her than any local kid in that two mule town.
