Can you describe a little more about your setup? What device/OS are you using? What process are you using to merge the layers?
I tried merging two vector layers on my iPad (Fresco v 6.6.1), and also on Windows 11 (Fresco v 5.5.5 and, after updating, v 5.7.0), and the results were that the merged layer retained its vector properties. To merge the layers, I used the three dots menu to "Select multiple," chose my two layers, and again from the same menu, "Merge selected."
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