The reason why Red Bull – therefore Toro Rosso as well – took so much time developing an interim version of the double diffuser was because of the RB5's different type of rear suspension. It's unique pullrod-based design made it quite difficult to incorporate a revised diffuser, as Newey & Co. have spent many late hours in order to get it done by mid-May.
Although no official announcement was made by the British outfit, it seems Vettel has confirmed the new feature when talking to an Austrian television broadcast. Whether Red Bull will go ahead and use it from as early as the Monaco GP, or wait until the next round, in Turkey, is yet to be seen.
As revealed by the circuit's characteristics, using a rear diffuser on the streets of Monaco is likely to count less than expected, as this is not a high-downforce track. If using the double diffuser, the team would more likely do it so they can evaluate the data ahead of the more important race in Istanbul (when the new design is expected to provide some sort of an advantage as compared to the initial RB5).
“But if the diffuser helps us, then it will also help in Monaco,” confirmed Vettel for ORF programme Sport am Sonntag.