You did it wrong with the vice, I mean the way you positioned it. The clamps should extend beyond the edge of the table so that you can hold something vertically like a long pipe or whatever. In the position like this, you don't have too much room for manoeuvring.
Still, I would advice to fix a 3 mm metal sheet on the surface of the table. That would add additional weight to the construction making it more solid and as well metal sheet is a great surface to work on. I doesn't get scratched, pitied or whatever. It is easy to wipe the spilt oil from the table. It works great for me on my bench and I also use it for clamping the ground from my welder - although rarely.
as above with the vice, plus if the bolts go through the metal of the table rather than just the wood it's a bit more solid.
If you place some decent thickness plate on top of the bench then you don't want to cover it with spatter when you weld as it's quite expensive stuff, so add some thinner disposable sheet on top...a bit of dexion shelving or old washing machine side panels, etc.
Use it for a while, grind it off occassionally, then when it gets too rough throw it out and put another sheet on there.
realized that with the vice once i had mounted it, it is mounted to its own metal sub frame and moving will be an awkward job so it will do for now it is just for the car stuff so i think it will ok for now. I will be making a seperate table for welding totally made of metal, this one is just for general use though i may metal plate half of it!