You won't be able to view visualizations directly within VS Code - what I'd recommend doing is saving your visualization and then opening it on QIIME 2 View. If you are using the terminal within VS Code, you can also run the following command to open up your viz in QIIME 2 View:
qiime tools view visualization-file-name.qzv
Where visualization-file-name.qzv is whatever name you allocated for your visualization when saving it to your machine.