An additional step for such a loose weave (= knit) is to attach the feature/fashion button with a small square of matching felt and the plain button on the inside of the bolero. Then, using a sewing needle large enough to accommodate either embroidery floss or #8 pearl cotton, stitch through all four layers at least four times.
From the outside in, here are the four layers:
Fashion button - knitted bolero - matching felt square - small button
An extra "security" step I recently learned is to sew the button down completely differently from the way I've always done it.
Original: Make an X with the stitch pattern through the four holes on the button.
Improved: Sew through two holes vertically or horizontally next to one another four times. Fasten thread off. Sew through the remaining two holes four times. Fasten thread off.
With the improved method, you won't lose the button if one side's threads come loose, whereas with the original method, that's a strong possibility.