I didn't find anything specific to 18215 and the 'Unknown Great Plains field subtype' is a generic message without knowing what the value references.
The fact that the integration runs for about 450 records and then stops, throwing this error, makes me think it is pretty much a timeout type of issue that we'll see with Integration Manager, especially when not using an eConnect adapter, which being a Vendor integration, it would be using a standard adapter, not eConnect.
You can try enabling logging for IM to see if that shows more information:
1. In the Integration Manager code directory, open the Microsoft.Dynamics.GP.IntegrationManager.exe.config file in NotePad and change the first and third lines of the following, to be a 1, such as this:
<switches>
<add name="IMTracingOn" value="1"/>
<add name="TraceGPScriptInstructions" value="0"/> (do not switch this one, it’s for Development)
<add name="TraceGPDexInstructions" value="1"/>
</switches>
Save changes and close this config file.
2. Right-click and choose 'Run As Administrator' when launching both Dynamics GP and Integration Manager. You must use 'Run As Administrator' on both in order for the IMTraceLog.txt file to be created.
3. Run the integration and re-create this error message about 'unknown Great Plains Field SubType' and '18215', then look for the IMTraceLog.txt file, which should be found in the Integration Manager root directory.
I'd also look in the Event Viewer logs where Dynamics GP and Integration Manager are installed, to see if those show any additional information.
Thanks,