After the huge shoot out in the hallway there’s no denying her involvement. Captain Antilles and Princess Leia are both just delaying Vader for as long as possible, hoping Artoo can get away and find Obi-Wan. As for the transmissions being beamed. We don’t know if Vader saw that soldier hand off that data tape. His mission as mandated by Tarkin is to mop up the space battle and prevent ships from escaping. So he could have been on his way to try to stop the Tantive from launching. Alternatively, as the Tantive was docked inside of Raddus’ ship, Vader could consider them both just one vessel and as we saw, the plans were beamed up in about four or five chunks simultaneously.
That is all well and good, but it isn’t exactly elegant in terms of explanation. It’s rather a bit of lawyerly technicalities that allow Leia’s speech to work rather than being an organic and believable lie.
It’s much more believable that Leia’s ship was on the edge of the battle for a brief moment to receive the plans before jumping back to hyperspace on her way to recruit Obi-wan. Why they stumbled over this simple plot device is anyone’s guess, perhaps just so Vader can have the fist shaking moment of failure.