You select the expense or asset account you need the transaction posted to, accounts are not assigned to vendors in QBO they way they can be in desktop

Resolution for 'assigned to vendors issue: 'When entering a new purchase ledger bill no expense accounts are linked' available: Yes.

The assigned to vendors 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 assigned to vendors issue yourself by looking for a resolution described in the article. If it is a complex assigned to vendors issue or you are unable to solve the issue, you may contact us by clicking here or by using other assigned to vendors support options.

For more information about resolving issues related to assigned to vendors in Quickbooks/QBO or to get support for the issue 'When entering a new purchase ledger bill no expense accounts are linked', click the link below: