The problem with vases is that they are covered in water for long periods. You can use some timbers as bowls, drinking vessles or whatever but they are only immersed for short periods. I've tried various things with vases, some say epoxy works but not all epoxy is suitable for long term immersion.
The risk isn't rotting, its staining of the outside as water seeps through and splitting as it expands and contracts.
I keep a selection of plastic containers that were headed for the recycle bin, things like shampoo bottles, to use as liners, and for small vases I have some plastic test tubes that are easily found on amzon/ebay etc. Glass is nicer but with plastic you can cut it down to the length you want and all you see is the rim anyway. It does give design constraints but you can get 'narrow neck' by hollowing from the bottom and inserting a disc.
Pictures follow, one lined from underneath with a cut down 330ml coke or beer can and one specimen vase (for single bloom) with a simple 16mm drilled hole and a plastiic test tube liner.
