If it would cost $X to develop this system inhouse, you could write down the requirements and offer the vendor to add the missing features to their product on N

If it would cost $X to develop this system inhouse, you could write down the requirements and offer the vendor to add the missing features to their product on NRE[1] basis for the fraction of $X (including support and warranty). NRE means that the IP remain their, and they will be able to amortize the costs across multiple clients. Assuming that this vendor is unresponsive because they're not incentivized enough (e.g. fixed pricing or SW licensing fees), not because they're incompetent. Also, most software projects fail[2], especially inhouse ones. I would expect vendors to have a much higher "not fail" rate (but not necessarily "success"). -- [1] Non-Recurring Engineering [2] stagnation is also considered failure