Vienna is 1 hour ahead of Reykjavik
Best overlap: 8:00 AM to 5:00 PM in Reykjavik is 9:00 AM to 6:00 PM in Vienna.
Book within 8:00 AM to 5:00 PM Reykjavik time (9:00 AM to 6:00 PM in Vienna) and both teams will be at work. That's a roomy 9 hours of common time each day, so finding a slot that suits everyone is rarely a problem. Note that Vienna follows daylight saving while Reykjavik stays put, so the times above drift by an hour seasonally.