The apex
namespace doesn't work in Aura. Inside a community, in the Builder, you can just click on Components, then add a Visualforce component, and specify the page in the component configuration. There's no need to write a custom component.
However, if you still wanted this approach, you'd want to use something like:
<iframe src="/communityprefix/apex/wallmart">
</iframe>
You need to make sure that the Guest User has access (Setup | User Interface | Sites and Domains | Sites | | Public Access Settings | Visualforce Pages), and make sure that the page allows itself to be framed.