Ghost wedding, what’s the benefit??

Is your friend Vietnamese of Chinese heritage?  Ghost wedding is a Chinese custom that has been banned by the Chinese government a long time ago, before I was born, but is still practiced every now and then, albeit illegally.

That evil custom has never had a place in Vietnamese culture.

They’re both Vietnamese, the wedding consisted of an introduction to the grand parents (deceased) to seek the blessings. Do viet wedding use a monk or priest normally?

Oh.  That doesn't sound like Ghost Wedding, which is the ceremony between two dead people or one dead and one alive person.   A macabre to (supposedly) ensure all the good things for the future including the right to inherit.  You can read about it on the Internet.

If your friend and his bride were both alive and kicking at the time of the wedding, then it's not a Ghost Wedding.  He must have used the wrong term.

During the first part of a regular wedding, after the groom and his family arrive at the bride's house with the offerings, he and his bride would light incenses at the altar of the bride's ancestors, kneel down to pay their respect and ask for blessing as he's now part of her family. 

On the Buddhist side, there's no need for a monk to conduct that part of the ceremony unless the bride's family insists (to ease the effect of a "bad" age match*, bad timing, bad one thing or another). 

On the Catholic side, there's no need for a priest to present at the house either because the ceremony must be held at the church.  The bride's family may ask their local priest to be there anyway to give his blessing just because.

*Bad age match doesn't mean a big age gap but the years and the elements under which the two persons were born.  The cat and the rooster, the buffalo and the goat, the rat and the horse, the dragon and the dog, the tiger and the monkey, the snake and the pig, metal and wood, wood and earth, earth and water, water and fire, fire and metal, those combinations are bad matches, generally speaking.