Yes, when you don't take the outbound you'll have the inbound cancelled. Only route I can think to use voucher 1 [other than just doing the whole trip in spring] is two nested trips:

March:
Outbound on vouchers booking 1 UK-US
Outbound booking 2 US-UK

October:
Inbound booking 2 UK-US
Inbound on vouchers booking 1 US-UK

They don't need to be the same place, so you could do NY and somewhere else. Of course, you may not want to do this from a financial POV.