I want to create new look on flower vases.
What look are you considering? This can be spray-painted now with the new innovative spray paint formulations with both paint and primer combined.
Paint, tape, decoupage are all options. Here's a link with more options:
You can paint them with an opaque paint. Decoupage is another option. Paint large gold dots on them.
The answer depends on whether your vases are clear or opaque. Clear vases are good for more types of paint effects, such as antique mercury mirror glass: https://www.17apart.com/2012/05/how-to-diy-antiqued-mercury-mirror.html This can be applied to either the inside or outside of glass. Paint the inside of a vase with enamel paint using either a pour technique like this: https://www.beaglesandbargains.com/diy-upcycled-painted-glass-vase/ or using an artist brush to apply the paint. Using a brush often requires a second coat that will obscure brush marks.
Alcohol inks give a transparent watercolor effect. There are numerous techniques so here are videos: https://www.youtube.com/results?search_query=alcohol+ink+on+glass Alcohol ink MUST be clear-coated for protection as it can be dissolved by water, alcohol and acetone. Here is a link showing one artist's preferred materials for sealing: https://alcoholink.community/how-to-seal-alcohol-ink-art/
Consider using Plaid FolkArt peel & stick stencils as design guides. Pébéo makes Vitrael transparent and opalescent paint that is water resistant once dry. They also make Vitrea 160 which is hand washable after baking the art object in your oven. https://plaidonline.com/Plaid/files/a8/a87bee74-8ac8-44df-86fe-a66bc6b78b7b.jpg