According to the documentation here
http://wiki.videolan.org/Documentation:WebPlugin
one can set the seek position by using
axVLCPlugin21.input.time = 20000; // in milliseconds
There is more information there on getting the total length in milliseconds.
And if it doesn't show up in your auto-generated COM wrapper, ditch it altogether and roll your own. It's not too difficult to make your own inheritor of AxHost and use a "dynamic" (the .NET 4 keyword) OCX object.