Fool the guards in front of the coffee houses to get access to the blueprints.
Assign your Venice disguise as a power-up, don it and approach the guard. To get clearance, repeat the button sequence Bentley shows you. Once the guard leaves and Sly takes his place, control will switch to Bentley, inside the coffee house.
Once inside, hop over the laser security and head for the painting. Search for numbers hidden in the painting. Once you do, enter the combination 4-7-9, and you'll get the first blueprint. Leave and repeat for the second coffee house, avoiding the spotlights. The combination for that safe is 9-6-8. Leave the coffee house, and control will once again switch to Sly.
Get to the third coffee house and say the code, but the guard blocking the door, Tony B., won't let you take over unless you know his nickname. Talk to two other guards in disguise; the second guard you talk to will know his nickname. Head back, say the password and nickname to Tony, and he will leave, allowing Sly to take over and Bentley to head inside. Control will once again switch to Bentley.
Avoid the spotlights and lasers, crack the art decryption lock (code is 1-1-3) and get out fast! An alarm will trigger once you take the blueprint. Head back outside and rush back to the safe house. Once you get inside, you complete the job!