Singular Concepts and Singular Negative Existentials

London Ed seems to be suggesting that we need irreducibly singular concepts (properties, propositional functions) if we are properly to analyze grammatically singular negative existence statements such as

1. Vulcan does not exist.

But why do we need to take 'Vulcan' to express a singular concept or haecceity property?  Why isn't the following an adequate analysis:

1A. The concept Small, intra-Mercurial planet whose existence explains the peculiarities of Mercury's orbit is not instantiated.

Note that the concept picked out by the italicized phrase is general not singular.  It is general even though only one individual instantiates it if any does.  The fact that different individuals instantiate it at different possible worlds suffices to make the concept general, not irreducibly singular.