The authors of the Viśiṣṭādvaita Vedānta school (a philosophical school flourishing in South India from the 10th c. onwards) claim that the whole world is made of God/brahman and that everything else is nothing but a qualification of Him/it. This philosophical concept, it will be immediately evident, crashes against the idea of a rigidly divided ontology, with substances being altogether different from qualities, as upheld in the more ancient school of Nyāya-Vaiśeṣika. In other words, the Nyāya-Vaiśeṣika world if seen from outside is similar to the world of today's folk ontology, the one influenced by scientism, while its structure resembles the one of Aristotle's ontology.
It is populated by subject-independent entities which are ontologically solid and persistent through time and to which qualities accrue which need them as their substrate. It goes without saying that this is a reciprocal distinction (substances are not qualities and qualities can never become substances), since it is grounded in an ontological difference (akin to Aristotle's ουσια). In other words, qualities cannot be further qualified by other qualities, since they cannot be their substrate (this leads to some complications, but I will not focus on the specific Nyāya-Vaiśeṣika answers now).
This scheme can not work if one wants to imagine the world as being constituted by just one reality (the brahman/God) with all the rest being an attribute of Him. In fact, this idea implies that there is one substance (God), which is qualified by further things which, however, cannot be called qualities, such as human beings and other material entities. One of the main authors of Viśiṣṭādvaita Vedānta, Veṅkaṭanātha (1269--1368) is not afraid of stating explicitly that his school does not use the term 'quality' (guṇa) in a technical sense, like Nyāya did (see his Nyāyasiddhāñjana 4.4) and that for Viśiṣṭādvaita Vedānta everything can be considered a quality of something else when it specifies it.*
This, however, does not amount to say that qualities and substances are only subjective constructs. The ontological grounding is provided by God's existence as the world's foundation. The fact that all human beings are qualifications (viśeṣaṇa) of Him is not a subjective construct, since it is rather a state of affairs which exists independently of all subjective minds apart from God's one.
This brings us to the next step, namely, the importance of God's existence to ground the world. Given that Viśiṣṭādvaita authors have given up the subject-independent ontology of Nyāya-Vaiśeṣika, they need another way to ground the objective existence of the world and this cannot be achieved but through God. He is ultimately existent and therefore we can opt for being and avoid plunging into nihilism. But what tells us that the world as we see it is also subject-independent? The fact that it is conceived by God. It is a content within God's knowledge. Thus, the main thing to be analysed for Viśiṣṭādvaita becomes the status of God's knowledge. Knowledge is said, in Nyāyasiddhāñjana 4 and before that in Rāmānuja's Śrībhāṣya ad 2.2.27, to be a substance. This seems a daring statement in a context which was used to the idea that cognitions are rather qualities of the self. Rāmānuja does not want, in fact, to deny that cognitions are qualities of the self, he only wants to state that they are also substances. They are, therefore, not kevalaguṇas 'sheer qualities' but rather dravyātmakaguṇas 'qualifications being substances'.
*As a homage to Veṅkaṭanātha I will also not try to render the Viśiṣṭādvaita guṇas with a term different than the one I use for the Nyāya guṇas. Viśiṣṭādvaita authors are not talking about other types of qualifications, they rather claim that what Nyāya authors call qualities are indeed not necessarily a different class from substances.
(a similar version of this post has been cross-posted on my personal blog)