We accept the vouchers issued by NS International itself: the paper Compensation Voucher and the Eurostar to Belgium/France (formerly Thalys) Voucher.
NS International cannot accept the digital Eurostar to Belgium/France (formerly Thalys) e-Voucher. The link in the email that you have received from Eurostar to Belgium/France (formerly Thalys) will allow you to access your e-Vouchers and use them directly when you book at Eurostar.com.
You can exchange other vouchers with the individual railway company that issued your voucher.