what do you mean for a few hours? like just to show your friends? goldfish are ridiculously hardy so I'm sure it won't hurt it unless the water in the vase has chemicals like chlorine or certain plant foods or detergents in it. maybe if you wash out the vase and dechlorinate the water and make sure its not uber cold it would be fine.
What kind of flower? Roses. DEFINITELY NO!!! Thorns of course. Being valetines day I thought you might be doing this. I would probably vote against this. To have a chance even small one for the sake of showing your friends ect is not a risk I would take. I have 3 goldfish in a 10 gal tank. It's amazing how they have their own personalitys. One will nip at my finger slightly if I put it on top of water while the other ones are amazingly curious. I had them for about 1 year now which is a record for me.. Most of my fish went to the "great aquarium in the sky".
I have seen cool aquatic plants and goldifish in large 1.5 + gallon vases and its magnificent... like JI'm said don't put them in a vase with thorny plants and don't put any plant food in the vase. preferably don't have it spend its whole life in the vase
I would definitely not recommend you doing this. Goldfish are very messy fish and really need at least 10 gallons for each goldfish. Housing them in a 1.5 gallon vase with cut flowers in the top is cruel in my opinion. Not only is the vase too small, but with cut flowers in it, they have even less swimming space and their oxygen availability it limited because the vase doesn't have much air/water contact.