Do you have any casting experience? If not, it will be very hard to get a nice and clean cast without proper supplies and experience. For a vase, I would definitely suggest following an investment casting style, but I would not suggest getting the print done in Nylon if you are going to cast. PLA is generally the go-to for investment casting, as it off puts the least noxious fumes when it is burnt out of the mold. To cast in metal, you will need a kiln, a foundry, a vacuum pump, metal ingots and mold making supplies.
Plating is a bit easier, you just need conductive paint, metal solution and a bar of raw metal to use as your anode, as well as a power supply. You can also have this done by a commercial service for less cost than casting would be. RePliForm is one of the ones that comes to mind for plating.
In terms of cost, plating will definitely be cheaper. And in terms of durability, most things you use now are plated metals. It’s very expensive to make things like cutlery and decorative pieces entirely out of a precious metal, so it’s very common for something to be made of steel and plated in silver for example. Durability of your piece will depend on what you plate it with and how thick the layer is, and what the base plastic is.