Having never used SX1 myself, but have used VST 5/32 for several years. I’m going to just guess…I believe you need to contact Steinberg and get your Cubase VST license put on a USB dongle to play along with SX together in the same machine. SX uses a USB dongle I’m assuming…? I don’t believe you need to uninstall your VST version 1st. I’ve never heard of anyone using the old (blue for VST 5/32) LTP dongle with a USB dongle together and working myself. My buddy has VST 5/32 & SX3, but they’re installed on different computers.
But hey, if you have both versions, you can make use of both on two machines for an inexpensive powerful combination of resources. And if you have VST 5/32 with the last update, you’ll have VST System Link to sync them together. Not your question, just pointing out a capability if interested.