England is a tough one indeed. For England, you'd think immediately "flyhalf" as they function through him so much traditionally and seem to pick one guy who's their starter for a 7-year stretch or so. I would actually put Owen Farrell there for England, but it's a bit awkward as we've just seen two other flyhalves play well on a number of levels in NZ just some months ago. But still, over longer stretches, Owen Farrell is quite "vital", isn't he, to England.
B.Vunipola is gigantic and very useful but some would rather pick Ben Morgan, I may be one of those in fact. I don't know England flanker situation, but I'm sure England fans don't consider Robshaw or Tom Wood as "vital", despite their consistent, often prolific work in every game. Don't see em much, but they work their butts off each time.
Is Dan Cole vital ? Don't think so, he may not be England's best scrummager and isn't a master in the loose either. David Wilson's there.
Hartley ? Don't think so. Youngs is fine.
I do think Mike Brown is vital to England, along with Farrell. I think we can add the second row with Launchbury and Lawes, or at least one of those. And so far Yarde is the best winger we've seen from England easily so, him too.
The center position is similar to France's, very deep, so many good combinations to try out, but we probably have Fofana as a Vital 5. For England, I don't think any of 36, Burrell, Eastmond or even Tuilagi (as seen during the last 6N) are vital.
Many England fans mention Corbs, and he's sort of the second coming of Christ the way he's been talked about, but we just can't put him in the Vital 5 as we haven't seen him play int'l test ball in too long a while. Maybe he's just not the dominant scrummager we once saw, and with the new rules, form, etc...
So in conclusion, I'd say, I may be wrong: Mike Brown, Yarde, Farrell, Lawes, Launchbury.