Hi, Harriet, I think Spaniards are in general much less likely to speak English outside the big cities than lots of other places. And I think you will find fewer English speakers on the Vdlp than on the Camino -- and I'm referring to both walkers and people in shops, restaurants, etc.
I speak Spanish, so I really can't give you a good feel for how hard it will be to walk with no Spanish, but I did meet people on the Vdlp who spoke little or no Spanish and they were doing fine. I think you will find that many people are eager to help you when you're in a pinch, and hand gestures with an occasional word can be very communicative! That said, I think some sort of dictionary or electronic translator would help a lot. A German man I met on the Vdlp who spoke little English and no Spanish found it very helpful for talking with me and talking with the Spaniards.
I found, when I walked last May, the overwhelmingly predominant language on the Vdlp was German (or something close to German, like Dutch, Swiss German, Flemish, etc). I remember eating a communal dinner at an albergue when a Spanish couple walked in -- they came and joined us and I remember one of them telling me he felt like a foreigner in Spain because he had a hard time finding other peregrinos who spoke Spanish.
I am pretty confident that the language issue will not be a big impediment on your walk, and the Vdlp is just amazing, well worth a little linguistic struggle.
Buen camino, Laurie