The coverage start date will be the requested date on the application - this must be a future date due to the quoting process; otherwise, it is when you accept Vitality's health insurance quote. If you've requested a quote via Kota but have not yet received an answer, or have received a quote but didn't confirm it to us, your employees won't be protected yet.
Once the quote is confirmed by you, your staff (the ones you've added to Kota and to this benefit - which should be all of your UK employees for compliance purposes) will be insured. No action is required from your employees' end. They will simply get an email from Vitality informing them they are now part of your health insurance plan.