what is vital wheat gluten and why do I need it

Emma already gave a great explanation about what vital wheat gluten is. And to partly answer your question, you don't really need it. If you want a more elastic dough, a higher protein flour of ~14% (sold as bread flour in the grocery store) will do the trick. But a quality AP flour (protein in the range of 10-12%) will work beautifully for most breads--and the lower gluten/protein percentage means the breads are less chewy. Bread flour (or spiking AP flour with a little vital wheat gluten) can be helpful but not required when making breads that contain high percentages of weaker flours (think spelt or rye) or lots of additions (like almonds in your case).

Agree with what Emma said. I used to like to add it to my challah dough back when I made it every week. It gives bread that "stretchiness" as opposed to a short, crumbly texture.

Hi Mary! The Bob's Red Mill website explains this ingredient really well: "Vital Wheat Gluten is the natural protein found in wheat. A small amount added to yeast bread recipes improves the texture and elasticity of the dough. Vital Wheat Gluten can also be used to make a vegetarian meat substitute known as seitan." You should be able to find it at any Whole Foods, if there's one near you, or a local health/natural food store.