In your comment you have written
the statement "all cell phones in the room are turned off" will be true whenever there are no cell phones in the room. In this case, the statement "all cell phones in the room are turned on" would also be vacuously true
based on this fact , can't we say that "the statement V1 is not a clique is also vacuously true when there are no pairs in V1" ?
No, you can't say that if you want to say something based on the above fact then you have to go to the definition of clique.
S1 : V1 is a clique.
We can write S1 as
S1 : V1 is a subgraph of G such that all pair of vertices in V1 are adjacent to each other.
Now, if you want to frame a sentence similar to " all cell phones are turned on "
That will be
S2 : V1 is a subgraph of G such that all pair of vertices in V1 are not adjacent to each other.
That is
S2 : V1 is an independent set.
S2 can be vacuously true. (which I have mentioned in my answer.)
And since singleton vertex is a clique so edgeless graphs contain cliques.