I don't recall saying consultant vs non-consultant, but the responses in the thread have addressed the question of who should interface with a vendor.
I feel like I should provide some context for how some vendors operate to get a better idea for the level of vendor involvement and who the vendor wants to work with.
It depends on the vendor, and who the customer is. For instance, some vendors are 100% channel sales (Datto I think fell in here) and a customer outright can't buy them directly.
Most larger vendors DO NOT WANT to talk/sell to smaller customers directly (It's too expensive, as they pay too good of benefits, and too high of compensation to their salespeople to scale down to small accounts that because they only sell their products can't form a meaningful relationship). There typically are 4 "buckets" for products.
a. Retail sales for VERY low-value non-complicated sale items that a website can sell. These products don't require sizing assistance or are pretty simple. Think an ethernet patch cable.
b. More complicated items on smaller deals that intend to be 100% channeled in sales (You don't want this stuff sold by Amazon as the customer will likely buy the wrong SKU, or screw upsizing). Note, the vendor may offer a direct model but will often have "cannon fodder" class salespeople in this space, and generally will even charge more for going direct. A VAR is your best bet here. Think someone buying 3 servers, or 20 laptops, or a single palo alto firewall for a SMB. all services are going to be VAR partner led when possible beyond post-sales support escalations. Also in these smaller accounts it's expected that the VAR/MSP is more than likely going to know the needs potentially better than the customer does.
c. larger enterprise deals where the VAR is still involved but the vendor takes some leadership because the account is big enough to matter, or the vendor wants a strategic presence in this account. The paper may shift to being run by the vendor at the higher end of this, with a small revenue share back to the VAR who brought this deal to them for the life of this deal. Think ELA's, 100 site MLPS circuit deals etc. services might be delivered by either the partner or the vendor at this stage.
d. Direct only deals. These are sometimes called "named accounts" and the vendor will 100% run paper directly. A partner of record might get 3% of the deal if they are lucky, or be subcontracted if they are a marque support partner with tons of certifications.
Others can comment but sales teams tend to be organized around these different groups Example:
Commercial-1 Smallest accounts and people who haven't bought anything in 5 years from you. These are called "Whitespace accounts" and you basically have people trying to get a meeting with hundreds of these in a territory or verticle and seeing if they can find some gold and get people with a low priced entry solution. ALL sales will be inside teams at this scale with VAR's or MSPs type shops doing any in person meetings.
Commercial-2 Slightly larger accounts. Might have spent a few thousand, but there isn't a strategic or lucrative relationship. You might have a field team at this point but they will likely cover hundreds of accounts still.
Midsized Accounts - Still larger. They will likely have some clue who their account team is, but still rely on a VAR for most day to day stuff.
Large Enterprise - Big names you recognize. These accounts will have teams who might have only 5-10 customers. Alignment on this is going to be tied to geograhpy still more than likely.
Globals - Account teams will be in some cases 1:1, or if there is a specific industry (Say automakers, or oil gas) you might have a team in a city (like Houston) whose job is to wrangle these guys. The Cxx levels of the vendor likely have strong relationships with these accounts and for a software vendor these accounts could be spending 9 figures at a time, or for hardware companies 10.