QB Issue Resolution:

No they should be added – but if any have exact duplicate names there may be a problem and some info could get merged.  I would first export your existing list and compare it to the list you hope to import – then eliminate any duplicates from the new list and import

Resolution for Issue 'If I import a vendor file from an acquired company into my QBs will my original vendors be deleted?' available: Yes (Solved).
Source: Intuit Community forum.

If I import a vendor file from an acquired company into my QBs will my original vendors be deleted?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for acquired company
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