The voucher would represent the provision of free hospitality to someone other than an employee and it would therefore be treated as ‘business entertainment’ by HM Revenue & Customs. Unfortunately, this means that any input tax you incur on providing the “supply” will be disallowed.
In the case of a supply of catering it is likely that the majority of the components used to make up the meal will be zero-rated foodstuffs, but you should disallow the recovery of input VAT incurred on any standard-rated items given away, such as the alcohol or ice-cream for the dessert.
The fact you have chosen to donate a proportion of the bar takings does not reduce your VAT liability and output tax should still be accounted for on the full amount of the bar takings. VAT exemption does exist for certain charitable fund-raising events by charities and other eligible bodies where they are advertised as such.
More information on this can be provided from your local TaxAssist Accountant.