Write vm_run_wat(wat) -- the top-level entry point. Split lines, scan for functions, find main, run it.

fn vm_run_wat(wat: []const u8) i64 {
    vm_split(wat);
    vm_scan_functions();
    vm_sp = 0;
    vm_local_count = 0;
    vm_block_depth = 0;
    vm_call_depth = 0;

    // find main
    for (0..vm_fn_count) |i| {
        if (streq(vm_fn_names[i], "main")) {
            return vm_run_at(vm_fn_start[i], vm_fn_end[i]);
        }
    }
    return 0;
}

Test it with hand-written WAT:

    const result: i64 = vm_run_wat(
        \\(func (export "main") (result i64)
        \\  i64.const 3
        \\  i64.const 5
        \\  i64.add
        \\)
    );
    print("vm result: {d}\n", .{result});   // 8

If that prints 8, your VM runs WebAssembly. Not all of WebAssembly -- just the subset our compiler produces. But it's enough.