So I guess u r aware the 15% no cap voucher is usually applicable to the 10% off items that you see which are currently offered by sellers right? From previous promo/campaign, very very likely they will have this 15% cashback. Just make sure the item have enough stock.

I think u can use the existing one too as it will auto convert to 15% coins cashback with no max limit. But there's always additional 15% coins cashback you can claim to use on the day itself. I guess no harm waiting unless you need the item urgently (provided the item is still available & seller is still participating in the 10% campaign). And even the 15% no max limit coins cashback is unavailable (which is unlikely), you can always apply the 10% voucher.

Go collect the 15% coins cashback no max limit voucher first, although I think the 10% one will likely become 15% by then. It will only be valid at 5pm so set your alarm! And yes, Shopee pay only.