If I play with another wiever ?

[OOgabrielleOO]

If I play with another wiever (eg Singularity) I do not have the opportunity to put my products on the market place after it is migration to VMM?

[Amethyst Jetaime]

The SL viewer that supports VMM is a project viewer in beta stage and will soon be in full release.  At this point it is the only viewer you can use with VMM.  However at least some if not all third party viewers will adopt the code for VMM and support it in their future releases.  You will need to check with the people behind Singularity to see what their plans are. 

In the meantime you will need to download the project viewer for VMM to add or change things in your store.