VAERS has a useful purpose, but in high profile situations such as this the data is often more reflective of data collection issues rather than actual effects of vaccination.
In addition to the reasons mentioned in another answer (people are more diligent about reporting incidents after the COVID vaccines because they are new and notable), note that VAERS has a literal form on a webpage where any member of the public can submit a report with no real verification/validation. This form has a notice that it is illegal to deliberately submit false reports, but since it is a form on the internet of course people do anyway. (As well as probably a number of people who intend to submit a correct report but have accidentally entered something incorrectly, etc.)
Looking at your source of OpenVaers, there is an item in their FAQ:
We do not change, modify or vet data. We take the downloads, upload them to our server and put a different face on them so they are easier to browse and get quick accurate info from. There are mistakes in the data (impossible dates are usually the most obvious), clearly, but we leave it as we get it.
And a very, very quick glance at their data confirms this: from an eyeball estimate, more than 7,500 people were reported as dying from the COVID vaccine between 1990 and 2019. Since the COVID vaccine didn't exist until 2020, this is a very easy example of the data quality and reliability of reporting here.
Additionally, the actual VAERS has a disclaimer that you have to click "I agree" to in order to access the data which elaborates on this. It reads as follows (reproducing in entirety because it's important, also as it's US gov't work it is not copyrighted):
VAERS accepts reports of adverse events and reactions that occur following vaccination. Healthcare providers, vaccine manufacturers, and the public can submit reports to VAERS. While very important in monitoring vaccine safety, VAERS reports alone cannot be used to determine if a vaccine caused or contributed to an adverse event or illness. The reports may contain information that is incomplete, inaccurate, coincidental, or unverifiable. Most reports to VAERS are voluntary, which means they are subject to biases. This creates specific limitations on how the data can be used scientifically. Data from VAERS reports should always be interpreted with these limitations in mind.
The strengths of VAERS are that it is national in scope and can quickly provide an early warning of a safety problem with a vaccine.
As part of CDC and FDA's multi-system approach to post-licensure vaccine safety monitoring, VAERS is designed to rapidly detect unusual or unexpected patterns of adverse events, also known as "safety signals." If a safety signal is found in VAERS, further studies can be done in safety systems such as the CDC's Vaccine Safety Datalink (VSD) or the Clinical Immunization Safety Assessment (CISA) project. These systems do not have the same limitations as VAERS, and can better assess health risks and possible connections between adverse events and a vaccine.
Key considerations and limitations of VAERS data:
- Vaccine providers are encouraged to report any clinically significant health problem following vaccination to VAERS, whether or not they believe the vaccine was the cause.
- Reports may include incomplete, inaccurate, coincidental and unverified information.
- The number of reports alone cannot be interpreted or used to reach conclusions about the existence, severity, frequency, or rates of problems associated with vaccines.
- VAERS data are limited to vaccine adverse event reports received between 1990 and the most recent date for which data are available.
- VAERS data do not represent all known safety information for a vaccine and should be interpreted in the context of other scientific information.
VAERS data available to the public include only the initial report data to VAERS. Updated data which contains data from medical records and corrections reported during follow up are used by the government for analysis. However, for numerous reasons including data consistency, these amended data are not available to the public.