What you call a "drive number" is named "Disk Index" in WMI parlance. You shouldn't have to parse anything to establish relations between WMI classes.

You can only get a volume label from a Win32_LogicalDisk's VolumeName property. It should work for mounted VHD(X) as well. The following code browses all partitions (note a disk can have multiple partitions) and dumps the associated logical disks (note a partition can have multiple logical disks) volume names, if any:

foreach (var partition in new ManagementObjectSearcher("SELECT * FROM Win32_DiskPartition").Get())
{
    Console.WriteLine("Caption : " + partition["Caption"]);
    Console.WriteLine("Disk Index : " + partition["DiskIndex"]);

    // associate the partition with logical disk(s)
    foreach (var logicalDisk in new ManagementObjectSearcher(
        "ASSOCIATORS OF {Win32_DiskPartition.DeviceID='" +
        partition["DeviceID"] +
        "'} WHERE AssocClass=Win32_LogicalDiskToPartition").Get())
    {
        Console.WriteLine(" " + logicalDisk["VolumeName"]);
    }
}

If you want partitions and volume names for a given disk, just restrict the initial query, like this: SELECT * FROM Win32_DiskPartition WHERE DiskIndex=1 for example.