According to Shopify API terms we synchronize customers both ways.
From Shopify to VTMarketing
The happens automatically when new customers are created within Shopify and our systems receive the webhooks.
From VTMarketing to Shopify
Because our public app captures customer data via Shopify online store, we do sync back those recipients automatically once a day.
If you need to do a manual synchronization go to Users > Users and click synchronize. This will send back to Shopify new recipients added in the last 24 hours.