If you break the rules of zero differential DC input, it can no longer be a "virtual ground with 0 diff input and is no longer linear output. The same applies to AC but declines as gain reduces with rising frequency. Negative feedback provides this linear null input voltage as long as the output is in the linear range.
Remember that an Op Amp is a differential to single-ended converter.
The term "virtual ground" is somewhat of a misnomer or easily misunderstood. But if you understand ground is just a local reference voltage and that reference can be floating above any other isolated ground, then it is accurate. For Op Amps (OA) we prefer input bias near the mid-range, other times near ground if allowed and other times near Vcc, if allowed in datasheet. ( or often just Vin+/- = Vcc/2 )
If you think it only means you have 0V on either input, then it is incorrect.
Why do I say isolated?
Because the output impedance of an Op Amp (OA) is often effectively a million times lower than the input impedance of either input ( without feedback ) we say the input is isolated from the output until you add feedback.
What does it really mean ?
We know the open loop gain > 1e6 , thus the only time you have linear operation is when we consider the differential voltage is effectively zero.
It is better to consider it like a floating voltage source with a differential impedance of zero but a very high common mode impedance.
The other factor is the common mode input range is not always Vee to Vcc. Some other performance aspects are traded off when when an Op Amp is considered Rail-to Rail input, while others make it work to the full positive rail (Vcc) or the full negative rail (Vee) which could be ground or 0V.
We do not consider each input as a virtual ground because they aren't. We consider only the differential voltage and not the common mode input as a virtual differential ground. Thus for differential gain, we must consider balanced impedances on each input to avoid common mode errors (e.g. (CMRR) and input bias current offsets.
The OA is a high gain differential-to-single-ended output. Now in reality there is some common mode (CM) connection but at DC it is attenuated by the order of magnitude open loop gain of the OA which is often > 1 million or 1e6 or 120 dB. In datasheets this rejection ratio is called the "Common Mode Rejection Ratio" or CMRR. This means if you change the input DC reference Voltage to both Vin+ and Vin- AND the differential input voltage is still Zero meaning the output is not saturated, then the output is not affected by the DC common mode (CM) input.
The CM is rejected or "isolated" as long as the assumptions for linear input and output range are satisfied. This ratio declines with rising frequency due to the gain-bandwidth product limitations of OA with internal compensation.
If your Op Amp ever has a differential DC voltage more than the rated Vio input offset (uv~mV range), the output WILL be saturated at either supply rail. Then it no longer has ANY linear properties. Always check for DC bias to each input to see how they match and the effects of input current creating an offset.