It's great to know that the store is offering 30% discount. However, voucher is a mode of payment, it is not a product, hence a 20% discount off face value on top of 30% store discount (on the scenario that the store is having a 30% discount as mentioned above) makes the whole thing even a better deal .
Besides, if you're mothercare's VIP member, you get 10% off regular price item, hence a scenario on a normal price item with 20% off face value voucher would give you 10% (VIP membership) + 20% off voucher on your purchase.
The expiry is Apr 2014 and the vouchers are in $20 denomination.