Loading a preset directly into the VST

I can extract and store single presets from a soundbank on my harddisk (as .fxp) using Juce.
Also it’s possible to load such a preset from disk and apply it again to the VST at a certain preset slot using Juce.
What doesn’t work: having the VST open (in my case Sylenth1) and directly load such a preset from disk (without using Juce).
Either Juce generally doesn’t support storing presets in the format which the VST expects or i need to change my code when storing extracted presets from a sound bank:

    // instance is an AudioPluginInstance
    MemoryBlock mb1;
    VSTPluginFormat::saveToFXBFile(instance, mb1, false);
    File file1 (AppData::Paths::presetsPath + String(preset->itemId) + AppData::Presets::presetFileExtension);
    FileOutputStream out (file1);
    out.write(mb1.getData(), mb1.getSize());

Could someone please confirm or correct that? Thank you.

This will just keep appending data to the end of the file rather than overwriting it, you know?

No no it works fine: These 4 lines will generate a new File since preset->itemId is increased in a for loop.
I just shortened it for brevity.