It sounds like you could use any MIDI keyboard that has a DIN MIDI OUT port to send messages to the Roland VR-09, such as the Akai MPK225 or Roland Juno 2 that you have mentioned.
If you want to send messages from the Yamaha P-125 to the Roland VR-09, it sounds like a "USB MIDI Host" could be a solution. The purpose of a "USB MIDI Host" is to convert a USB type B port (the square shape) which sends MIDI messages into DIN MIDI ports. Then you could use a MIDI cable to connect to the Roland VR-09.
A recent thread discussed using a Yamaha P-125 with a USB MIDI Host
. In that thread, I said I believe you would need to find a USB MIDI Host that will work with USB hubs, because the Yamaha P-125's multiple USB functions will probably appear as a USB hub with multiple devices connected to it.
In a very generalized simplification, products with a USB port can be a computer "host" with a type A port (flat rectangle shape) or a peripheral "device" with a type B port (square shape). Simple USB connections will use a USB A to B cable to connect a host to a device, for example, connecting a computer (host) to a printer (device).
When converting USB MIDI to DIN MIDI, it matters which side the USB connection is on, host or device:
To connect a computer's USB A port to a DIN MIDI device, you need a USB MIDI interface, like the Roland UM-ONE mk2.
To connect a MIDI device's USB B port to another MIDI device's DIN MIDI ports (without using a computer as a go-between), you need a "USB MIDI Host".