Depending on the Storefront version you're running, you may want to Reset your email template and Save (take a copy of your template first before overriding).
The latest template should already include the following code near the the top that lists out voucher payments. Look for the following text (this is available in the default email Receipt or Invoice template). Notice it loops over the salesPayments node inside a for-each because there could be multiple payments (even multiple vouchers).
<td>{xsl:for-each select="in/salesOrder/salesPayments/salesPayment[transactionType = 1 or transactionType = 2]"} {xsl:value-of select="paymentMethodName"/} {xsl:value-of select="creditCardHint"/}{xsl:value-of select="paymentHint"/}{xsl:value-of select="voucherHint"/} {xsl:value-of select="formattedAmount" /}
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{/xsl:for-each}</td>
I tested a transaction and it emailed me a receipt with the following text:
Gift voucher VMBB $10.55