Use VestaCP built-in tool to generate a CSR
Save the gererated CSR file and private key file.
Hand in the CSR to Comodo and you will receive a zip package containing these four files. The last masked one is the signed SSL Certificate.
Input the content of the last file showed in the above picture in to "SSL Certificate" below.
Input the content of the private key you saved in Step 1 into "SSL Key" below
You need to make my your own bundle file from CRT files
The order is: 1->2->3 showed in the picture in Step 2
Linux or UNIX-like Operating Systems:
cat COMODORSADomainValidationSecureServerCA.crt COMODORSAAddTrustCA.crt UTNAddTrustSGCCA.crt AddTrustExternalCARoot.crt > yourDomain.ca-bundle
Windows or DOS:
copy COMODORSADomainValidationSecureServerCA.crt + COMODORSAAddTrustCA.crt + UTNAddTrustSGCCA.crt AddTrustExternalCARoot.crt yourDomain.ca-bundle
Input the content of yourDomain.ca-bundle you just generated into "SSL Certificate Authority / Intermediate".