When using an existing VPC there are some aspects that should be taken into account, as you’re deploying the solution into an existing VPC, can you make sure that your VPC has two private subnets in different Availability Zones for the database instances? These subnets require NAT gateways or NAT instances in their route tables, to allow the instances to download packages and software without exposing them to the internet. You will also need the domain name option configured in the DHCP options, as explained in the Amazon VPC documentation. You will be prompted for your VPC settings when you launch the QuickStart.
In the same way, looking for "Parameters for deploying WordPress High Availability by Bitnami into an existing VPC" in this guide, you can find some tables with all the required parameters.
Apart from that, here you can find the template.
Carlos R. Hernández