You can apply polyurethane to wood no problem. Trouble is, it is usually an oil base which will cause the stuff to soak into the wood and then you will not have a nice uniform glossy finish (the grains of the wood will be dull). That's why I put down a base coat of latex paint first then go over the top of that with the polyurethane - the latex will fill in those pores and prevent the top coat from soaking away. Sure, you can keep piling coat after coat of poly on raw wood and eventually it'll fill in, but the latex saves you all those intermediate steps.
The latex doesn't really make the base less solid. If you use a decent latex for the base coat it should stick just fine and would probably wear fairly good the way it is unless it has heavy stuff dragging over it. And in this case, the polyurethane will be the thing taking the brunt of the activity, which it can handle easily.
Hmmm, that really depends on a number of factors. What is the flooring in the other rooms which connect to the hallway? How ornate is the hallway? You don't want to ruin the look of the hallway by using vinyl flooring, especially if it is an entrance hallway. How much use will the hallway get? Are you planning on selling your house in the near future? Do you mind a cold floor? What material are you most comfortable installing?
If it would be up to me, I'd use tile for a very ornate entrance hallway especially if you were planning on selling your house in the next few years. You'll probably get a good return on investment there. For just a normal hallway between rooms, I'd probably go for laminate wood flooring. It wears decent, and installs dirt easy. Here in the US laminate flooring is a big selling feature too. For a back hallway or one that doesn't get seen much or takes heavy wear then vinyl would probably be a good choice.