Just noticed that the black bar is actually a scrollbar, seems that the VST2 reports slightly different size data to Reaper, so the Reaper plugin window is a bit too small.
Anyway, I think it would have been better to leave the resonator section out, it is kind of useless and inappropriate for a classic analog synth. Would have been better to use that GUI space to make two separate panels for the phaser and the chorus/flanger/ensemble, and to add a compressor panel.