Another critical part of the marketing process, the open house offers prospective buyers the chance to view houses in a low-pressure, "browsing" atmosphere. With that in mind, you shouldn't expect it to generate a sale, at least not directly. What you should look for is traffic, and calls for private showings in the days following an open house.
Open houses are always valuable, even if very few people show up. Such a situation can indicate that the price is too high; it may also lead you to look for ways to improve curb appeal.
The sale of the severed portion of vacant land would not be subject to GST/HST since it is not capital property used primarily (more than 50%) in a business and it is not being sold in the course of a business. However, if the land was subdivided into more than two parts, sales of the severed portions are not exempt unless the portion is sold to a relative (or to a former spouse or common-law partner) for their personal use.