I agree that @nhopton's answer solves the OP's problem as VMD is a cartographic product and not a routing one.
For what it's worth though, I have created two routeable networks using PgRouting and OS datasets. I have a full UK "quick and dirty" network made using the OS Strategi shapefiles and a more detailed network of Scottish roads from ITN (Integrated Transport Network). ITN was loaded using Snowflake's GoLoader to import the GML into PostGIS.
For the Strategi network I had it up and running in less than 30 minutes. Download the shapefiles from OS (https://www.ordnancesurvey.co.uk/opendatadownload/products.html) and extract the shapefiles from the zip file. Using QGIS I merged all the roads shapefiles into one large shapefile and then imported this into the PostGIS database (almost 500000 records for the whole UK). I then followed the pgrouting tutorial (http://docs.pgrouting.org/2.0/en/doc/src/tutorial/index.html) to build the network using pgrouting version 2.0. Version 2.0, I found, is a lot quicker than old version 1.0.x in building the topology.
For the ITN network I had to do a bit more work once the data was loaded into PostGIS. I have a couple of functions that generate the grade separation and one way street tables and then another that generates a network table combining the line segments and grade separation / one way tables. Using this table I build the topology and vertices table. This gives me a table I can route across but with errors. There are two areas that need attention after the topology has been built: line segments making up bridges need to be merged together to create a single segment and one way streets need to be checked for correctly digitised line direction.
Solving the first problem, I used an FME LineJoiner transformer to merge all the elevated sections into distinct segments (so up+across+down becomes just one line). The same thing could be done using PostGIS ST_LineMerge, I imagine. These new segments are then inserted into the network replacing the original segments. You can build the network topology again to check.
To fix the problem of one way streets going the wrong way (pgrouting uses digitised line direction) there are some helpful attributes in the ITN tables. The road links that are one way have a line orientation attribute so you can see if the one way goes with or against the digitised direction. It is easy then to select the one way streets that go against the flow. In QGIS I used the Swap Line Direction plugin to flip all the edges in one click (is there a PostGIS function to do this?). Save the table and rebuild the network topology.
Now, you should have bridges of single segments, one way streets all going the correct way so you just need to set up some costs. I set up distance and time costs on the segments and set very high reverse costs on one way streets to help ensure correct routing. You should now be good to go.
ITN has a heap of attribution that my method doesn't use. There are additional turn restrictions and road routing information that can be built in to create a very realistic model. Check the spec here.
