Not specifically for Vienna, though it's a gorgeous city with a good expat community.
Why was Berlin a disaster? I suppose it's about looking at what went wrong there and trying to avoid the same situation, especially since you'll have a 6 month old child again.
As someone who lived abroad for 18 months whilst my partner was working, I can only emphasise the importance of getting out there and being involved. I'd say it's nice to have a friend already there but be wary of leaning too much on one friendship. It will help for the beginning though.
Are there any expat groups for Vienna on Facebook? We had a very active expat community who used to organise trips to local festivals, canyoning trips etc. Very friendly and welcoming. I looked at internations but heard conflicting reports and didn't fancy paying for something that another expat group was doing better, for free.
When we were abroad last time, my wife often had to work away and I couldn't always go. I used to have a rule that I had to have a conversation with at least 5 people per day. Sometimes it was hard, sometimes super easy (especially since we took our dogs with us).
Also if your eldest will be at kindergarten, it should be fairly easy to meet other parents. I'm assuming you'll be looking at international schools? Plenty of summer fairs, school recitals, book sales.
What a fab opportunity! As someone who is currently 7 months pregnant and knows she could be living abroad this time next year (wife's work again), I'd be interested to know how you get on. Even though I've done it once without a baby in tow, I'm a bit nervous of feeling isolated as you described.
Sorry for mega (and not at all specific to Vienna) reply, hope vaguely helpful though.