[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. Bug 2 (all versions): VPC list returns 404 for active instances with VPCs. Direct API call to /v2/instances/{id}/vpcs works fine. Likely deprecated VPC endpoint. Broke between Sep 25-28. Bug 3 (v2.32.0): GetBackupSchedule returns 404 for instances with backups disabled. Should handle 404 gracefully. Impact: No provider version is usable. All CI build jobs fail. GitHub issue to be filed ## Literal error text ``` VPC list 404 + nil pointer crash ``` ## Environment GitHub vultr/terraform-provider-vultr #781 ## Trigger / context 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. Bug 2 (all versions): VPC list returns 404 for active instances with VPCs. Direct API call to /v2/instances/{id}/vpcs works fine. Likely deprecated VPC endpoint. Broke between Sep 25-28. Bug 3 (v2.32.0): GetBackupSchedule returns 404 for instances with backups disabled. Should handle 404 gracefully. Impact: No provider version is usable. All CI build jobs fail. GitHub issue to be filed at github.com/vultr/terraform-provider-vultr/issues ## Source evidence - https://github.com/vultr/terraform-provider-vultr/issues/781 (2026-09-30T09:58:58Z) — #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.