The Float Vertex Attribute texture operates similar to the Color Vertex Attribute texture node. The difference is that the Float Vertex Attribute produces a float value from the vertex paint data that you can use for Material attributes, such as Opacity. Add the corresponding vertex color set name to the Float Attr. 1 Colorset attribute found in the Object's Shape node under the Octane rollout, then clicking on Vertex Attributes > Float Attributes.
Figure 1: Adding the corresponding vertex color set name to the Float attributes in an object's Shape node