Half-Life 1 and all GoldSrc engine games use WAD files for textures. The simplest way to make one of those is to use a program called Wally, which you can download
here.
Vtf files contain more information than Half-Life uses, as WAD textures are made up of several 256 colour images - The texture itself, and mip images, which are lower resolution versions of the main image used when the player is far away to save rendering time.
What you need to do is open the vtf in an image editing program, crop or resize it so the dimensions are multiples of 16, save it as a normal image type (bmp, jpeg, png) and then import it into Wally.
I tend to open the image in MS Paint, select all and copy it, then open Wally and press ctrl+v. (copying it from Paint seems to be necessary, as images copied from more advanced image editing programs apparently contain more information than Wally is able to handle)
Once you've copied it into Wally, you'll be asked to name the texture (beginning the name with certain characters can give the texture different properties), and once that is done you can save it as a WAD. You can then put this WAD in the directory of the game or mod, and add it to Hammer.
Doing it this way is fine, but individual WAD files can contain multiple textures. To place all your textures in one file, use the WAD merge function in Wally, accessed through one of the tabs at the top of the program.
If you need any more information I can take screenshots to walk you through it later when I am at my computer.