Here are the T&C pasted from the link attached to the 2010 voucher...but I think they remain vague. My one way and mixed class comments were based on what I could actually book - I could not book Open Jaw On-Line and didn't want to call the agents to ask, as I felt they might have better things to do.

Redemption Voucher for a Companion Terms & Conditions

Earning a Redemption Voucher for a companion

You are only eligible to receive a Redemption Voucher for a companion if you are a member of the BA Executive Club and are the main account holder within a BA/Chase Credit Card account (a "Member").
Additional cardholders to the main Credit Card account are not eligible to receive a Redemption Voucher. Any amount spent on any supplementary Credit Card(s) of the main Credit Card account will count towards the main account holder’s Spend Target.
To be entitled to receive a Redemption Voucher the Member must have reached the required Spend Target within a 12 month period starting each year on the 01 January and ending on 31 December (the "Twelve Month Period") For the avoidance of doubt, if a customer opens a Credit Card account on any date following 01 January then they must have reached their Spend Target by 31 December of that year to be awarded a Redemption Voucher.
The qualifying Spend Target of the BA/Chase Credit Card is 30,000 USD (the "Spend Target"). Qualifying spend does not include cash withdrawals, interest, balance transfers and fees.
Only one Redemption Voucher can be earned in any Twelve Month Period. If the Member reaches their Spend Target within the Twelve Month Period then further spend will not accumulate towards another Redemption Voucher . Members can begin earning towards their next Redemption Voucher at the beginning of the next Twelve Month Period.
When the Member has reached the Spend Target an e-voucher ("Redemption Voucher ") will automatically be added to their BA Executive Club account. The Redemption Voucher will be valid from the date on which it appears on the Member’s Executive Club account and will be valid for 24 months.
Using the Redemption Voucher for a companion

The Redemption Voucher allows the main BA/Chase account holder, when making a return flight booking using BA Miles, to book another seat on the exact same journey for a companion without having to pay the BA Miles flight price for that Companion. Taxes, airline, government, or other separately charged fees and charges must be paid by Member redeeming the Redemption Voucher and is payable at time of booking.
The Redemption Voucher will be issued in the name of the main BA/Chase Credit card account holder, who must travel on any Redemption Voucher booking.
Redemption Vouchers may only be used for bookings, which are strictly subject to availability and BA offers no guarantee that Members will be able to book for a Companion on any specific flight.
A Member who has two valid Redemption Vouchers on their account has the option to redeem both of them simultaneously on one flight booking, the BA Miles flight price must be paid for the Member and an additional individual in order than 2 Companions can travel on that booking.
Rules and Restrictions

Redemption Vouchers can only be used for travel on reward flights listed in the Executive Club Redemption table operated by BA, in any cabin class, as long as the Member has sufficient BA Miles to complete the booking for their desired journey.
Companion Vouchers cannot be redeemed for travel with BA’s franchisees, code share partners or oneworld Alliance members.
Redemption Vouchers must be booked via BA channels only, these being the BA Call Centres and online at ba.com.
The Member and their Companions must travel together at all times and therefore must be booked onto the same flight and cabin class when travelling using a Redemption Voucher.
Bookings for Companions must be made at the same time as the Member makes their booking.
Companions do not have to be named within a BA Executive Club Household Account. Members can choose to take anyone as a Companion on their Redemption Voucher booking.
Outbound travel must take place before the expiry date stated on the Redemption Voucher however inbound travel may take place after the expiry date.
Redemption Vouchers are non-extendable, non-transferable and have no cash value. No BA Miles or other frequent traveller programme miles or points may be earned on Redemption Voucher travel.
Once travel using the Redemption Voucher(s) has been booked, name changes are not allowed.
If travel using a Redemption Voucher is cancelled within 24 hours of departure, the Companion Voucher will not be re-issued to the Member’s Executive Club account.
If travel using a Redemption Voucher is cancelled more than 24 hours before departure then the Redemption Voucher will be re-issued to the Member’s Executive Club account for the remainder of its original validity period.
BA reserves the right to amend these Terms and Conditions at any given time. Any amendments made to these Terms and Conditions will not affect the validity of any Redemption Vouchers which have already been issued.
Redemption Vouchers cannot be used in conjunction with any other offer or promotion, including Priority Reward (Gold Executive Club membership benefit). For more information on Priority Reward please visit ba.com.
Any use of the Redemption Voucher and/or use of BA Miles must be in accordance with the BA Executive Club Terms and Conditions. Please visit www.ba.com for more information.
Members and their Companion(s) will be carried by BA in accordance with its notice and Conditions of Contract (as stated on each ticket) and its general conditions of carriage for passengers and baggage (copies available on request).
Members must remain eligible for this promotion at the time of travel. BA and Chase reserve the right to withdraw any Redemption Voucher in the event that the Member is in breach of the terms of the BA/Chase Credit Card or the BA Executive Club Terms and Conditions.
Offer subject to availability and may be withdrawn at any time. Additional significant restrictions may apply.