I don't know / haven't check how this library is implemented.
Recommend to use dual framebuffer. Where one is in edit mode and other is drawn to display.
This will prevent flickering and screen tearing. Maybe need to create your own library
That fill command sould be fast. flickering can happen if next command take time or fill command is done by NIOS II softcore.
Is program stucture this kind?
//************
for(i=0; i<SensorCount; i++)
val = ReadSensor(i);
vdgfx.fillRect(0,0,640,480,vdgfx.White());
for(i=0; i<SensorCount; i++)
{
_ vdgfx.text.setCursor(150,350 + i32);_
_ vdgfx.println("Sensor %i value is %i", i, val);_
_}_
_//***********_
So print call come right after dispaly clear. Sensor read can take time and can cause flickering.