When you book is much less important than when you are planning on travelling. May is the "shoulder" season and rates are still discounted compared to the dead of summer. I personally find that flying in and out of VCE is actually one of the cheapest options. You may want to look into an open-ended ticket though if you are going to see all those places so that you don't have to backtrack, ie fly Vancouver-Venice and then London-Vancouver.
I am actually heading to Barcelona in May and scored an insane deal through United for business class the whole way for 10 nights in May for $1650, the deal is dead now, but there may be other options coming up soon. $1100 is pretty average for a flight price, make sure you try different days of the week etc to fly as there are sometimes huge differences.
I almost always book apartments through AirBNB now as opposed to booking hotels.
For example, this is where we are staying in Barcelona for $115/night:
https://www.airbnb.ca/rooms/790563
1000 sq ft, 2 full bedrooms, a proper kitchen, etc.
Mark