Can someone explain if you can use an engineer's vice (metalworking vice) to hold wood? Thanks.
You can use anything that will hold wood to hold wood. Over the centuries this has meant just pushing the wood against a nail or other projection from a bench or beam that you're working on (a method still in common use today all over the world1), wooden holdfasts made from a Y-shaped branching of a tree or bush and eventually the iron or steel versions with the same basic shape, various arrangements with ropes and wedges (sometimes together), V-shaped cutouts in a board (a "bird's mouth"), all-wood clamps (e.g. the original handscrews) and all-wood vices and their modern wood-and-metal counterparts2, and then the modern all-metal vices we know today. This can extend to any vice of any size, shape and type made from metal.
This perhaps isn't the best example since the vice is pretty tiny and the piece of wood quite large, but I wanted to use it first because that's George Orwell for those who don't recognise him, and how many of us know he liked to do woodwork when he could? :-)
So some much better examples to illustrate that this isn't something only an amateur does in a pinch, when they have no better option:
These are high end shotgun (££££) stocks being worked on in each of the images, including at Holland & Holland and Purdey, makers of some of the most expensive guns in the world.
Using what are, on paper, metalworking vices to work on wood has a very long history, probably stretching back to the very earliest metal vices (poss. the blacksmith-made leg vice).
Notes on use
Engineer's and mechanic's vices, even small ones, can produce huge clamping force, easily enough to dent some of the hardest woods without exerting yourself pushing on the tommy bar. Softer and medium-hard woods in including oak or beech can still be marred even if the vice is fitted with some of the commercially available 'soft jaws'.
So when working with wood if you don't want it dented it's usually important to at least use scrap wood pieces either side of the workpiece to protect it. But manipulating three pieces of wood each time the vice is adjusted is cumbersome and frustrating, so it's worth your time either lining the jaws with wood or hardboard3, or actually making your own soft jaws from wood4. You should probably also try using thick leather or felt as cushioning, although personally I find the 'spongy' grip unsatisfactory if I'm doing any job that puts force into the wood 5; be cautious using fabric as lining as believe it or not the fabric texture can be impressed into the wood surface!
Also you don't want things moving around as you work on them and when sawing or shaping by rasp, plane or chisel even a fairly heavy vice of 15-25kg (~30-55 lb) can walk along a work surface if it's not firmly fixed in place. So if your vice isn't heavy enough that it can't be budged, it should be bolted directly to the workbench or work surface, or bolted to a piece of stout wood (most use plywood or MDF these days) that is then clamped to the working surface, whether that's a workbench, a kitchen table or even a chair.
1 Q. v. the Japanese planing beam, rising bench stops set into many traditional European workbenches and any version old or new of the bench dog.
2 Including traditional leg vices, wagon vices and tail vices.
3 Inset magnets can make these easy and fast to swap in and out.
4 The wood linings or jaws don't have to be softer than the wood you're working with, but it can help further to prevent denting your workpieces.
5 It's both a little disconcerting and it steals energy, so the work takes longer.