1. You should have read access to your ntfs partitions out of the box with Ubuntu using ntfs-3g fs driver. So accessing and using files from the external won't be a problem. ntfs-3g also allows for write access, but it's not 100%. Point being, you can do it, but you may also encounter a problem along the way. As Done_Fishin mentioned, a FAT32 partition for both OSes to read/write to would be best.
2. I would look into
ardour 2,
audacity, and
energyXT. All are native Linux applications and, AFAIK, support VST. You may also want to look at Ubuntu Studio, which is a production optimized release of Ubuntu. You can at least get an idea of the software available and then install it in the base Ubuntu.
This might also be worth a look:
VST in ardour
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Arch64 ~ PekWM