Hi Usaghi,
If your refer to my previous phrase,
the supply is at high impedance, then the part is powered indirectly by the reference.
I meant your LDO output impedance. there is not boundary limits....
the problem is more simple and depends of your voltage reference maximum current.
the answer is the sum of the current driven to GND by the LDO and the AD5342 current consumption.
If your reference can drive both loads, you wont have problem (Say that I'd imagine that there more components sharing the Vdd net so you need to add the current consumption of all the components)
If not, you need to understand the current path
Hope it helps, let me know if something is not clear, please