I think it is fine to draw 10 mA - 50 mA from the same supply as feeds the AVdd on your nucleo board. You should to be careful not to inject any noise onto that line (as you're making an external connection which might couple to sources of noise).
But I also think the 500 mA spec of the regulator that feeds both Vdd and AVdd is probably not the limiting factor.
For a start, 500 mA is only achievable if the voltage drop across the regulator means that the power dissipation limit is not exceeded (power = volts x amps; more power means the regulator gets hotter as it can only lose heat at a certain rate).
And then any power-supplies involved in feeding that regulator must not be overloaded. They probably also feed other part of the board. If you're feeding power in from e.g. a 9V regulator, there's a regulator to drop the 9V down to 5V which might overheat if you draw too much current.
If you're taking power from the USB of your PC, the USB can only deliver 500 mA (and officially the 500 mA must be negotiated; many laptops can't deliver even that much).
Final comment: I think the title of your question is slightly confusing (in that you're not drawing current from the VddA pin, just from the same supply as also feeds VddA), and that might be why no-one else has replied to it.
Hope this helps,
Danish
