Ādvaita can easily incorporate Formless Brahman because they believe that Brahman is without attributes, and Form is an attribute.
On the other hand, Viśiṣṭādvaita believes in Realism (asti, asti), that everything including illusions are real, because the illusory object has some common properties with the thing imagined like snake and rope have some common properties and one can easily mistake snake for a rope in dim light or pitch dark.
This means that the attributes are also real and Brahman has attributes. There is a widespread misconception that anything with attributes cannot be formless.
How does Viśiṣṭādvaita incorporate formless Brahman, with Realism?