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HenriS, my initial suggestion would do just as you suggest, give you an Eb Trumpet pitched an octave below our normal 'Eb Trumpet.' When you press the valves down you are essentially creating a new natural trumpet with an overtone series lower than instruments initial one. So, press down the second valve on a Bb Trumpet and you've got a Trumpet in A and so on down. Press down all three valves, kick out the third slide for another half step and you've got a Trumpet in Eb.

I'm assuming Vaughan Williams just wanted one of the old big Eb/F trumpets in use around the turn of the century by Mahler, Bruckner, Strauss, et al. They would have been pitched in the correct octave to play all of those notes without any valves, you'll see especially Bruckner writing for them in between the C and F# below middle C, a range that our modern smaller trumpets can't play the written notes, but since the Eb and F trumpets transposed up, we can play sounding notes.

I hope that wasn't too confusing. The book I mentioned above, Norman Del Mar's Anatomy of the Orchestra has a good section explaining this a bit further in the section on Range - Trumpets, in my copy p 286. I'd recommend every player, conductor and composer should have a copy of this book, it's incredibly informative about all of the little things that pop up while playing in an orchestra, and is frequently hilarious.

Jason