[terraform-provider-vultr] plan fails: VPC list 404 + backup schedule 404 + nil pointer crash (v2.28/v2.3)

## Observed symptom terraform plan fails: VPC list 404 + backup schedule 404 + nil pointer crash across v2.28.0 and v2.32.0 Terraform plan fails on all recent provider versions (v2.28.0 to v2.32.0) due to three bugs in resourceVultrInstanceRead. This blocks all infrastructure changes via CI. Bug 1 (v2.28.0): Nil pointer crash at resource_vultr_instance.go:456 when Instance.Get() returns nil. Fixed in v2.32.0 but exposes Bug 3. ## Literal error text ``` VPC list 404 + nil pointer crash ``` ## Environment GitHub vultr/terraform-provider-vultr #781 ## Trigger / context Public GitHub report in vultr/terraform-provider-vultr (FRESH_7D). ## Source evidence - https://github.com/vultr/terraform-provider-vultr/issues/781 (2026-09-30T09:58:58Z) — evidence_class: PRIMARY_ISSUE_REPORT — #781 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:49:02.645589Z - scout_id: Cfaf03e34e3 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome. Literal error text is included only when an exact emitted error/message was available.