Massimo Salvadori’s definition of Socialism

Extract from page 4 of Massimo Salvadori’s Modern Socialism:

a movement centered on uncompromising opposition to capitalism, giving pre-eminence to the economic aspect of life, convinced that the end of capitalism will usher in an era of permanent prosperity, peace, and progress; aimed at achieving an egalitarian and brotherly society through the abolition of most or all individual ownership of property; strengthened by the certainty of possessing the truth and the key to happiness for all.

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Later on, a funny contradictory statement:

Socialists were clearer about what they were against, than about what they were for. Their energies were focused on discussion of capitalism and agitation against it. On the formulation of Socialism there is, in contrast, vagueness.
Socialists differed in what they intended by capitalism. For most, it was a vague general term including practically everything non-socialist.

So, Socialism is vague but it is not vague that it is against capitalism, the definition of which is vague too, since it depends on defining what is non-socialist.
What is not vague, then?